BY DG CLARK, JR GREENWELL, AA HARPER, ALISON M. SANKEY AND T. SCRATCHERD From ...

14 Pages·2006·1.4 MB·English

J. Physiol. (1967), 189,pp. 247-260 247
With 7 text-figures
Printed in Great Britain
THE ELECTRICAL PROPERTIES OF RESTING
AND SECRETING PANCREAS
BY D. G. CLARK, J. R. GREENWELL, A. A. HARPER,
ALISON M. SANKEY AND T. SCRATCHERD
From the Department ofPhysiology, Medical School, University of
Newcastle upon Tyne
(Received 15 July 1966)
SUMMARY
1. The electrical properties oftheresting and stimulated pancreas have
been studied in the anaesthetized cat. There is an inverse relationship
betweenthefrequencyofappliedalternatingcurrentandtheresistivityand
dielectric constantoftherestingpancreas. Theresistivityisalsoaffectedby
thefat contentofthegland. Theimpedancelocusofthepancreasissimilar
to that ofother tissues.
2. On intravenous injection of secretin there is a brief increase,
followed by a more marked decrease, in conductance and capacitance of
the pancreas. The phase ofdecreased conductance is relatedto the flow of
pancreatic juice. The decrease in conductance and the flow rate can be
characterized by two closely related empirical equations.
3. During secretin stimulation the sodium concentration in pancreatic
tissue increases, and the potassium concentration falls.
4. It is tentatively suggested that the decreased conductance across
the secreting gland is mostly due to swelling ofthe secretory cells and to
a minor degree is the result ofchanges in the composition ofthe secretion
in the pancreatic ducts.
5. Inthe anaesthetized cat the mean halflife ofthe secretory action of
secretin is 199 sec.
INTRODUCTION
There is an inverse relationship between the electrical resistivity of
tissues and cell suspensions to applied alternating current and the fre-
quencyofthe current, dependent onthe highresistance ofcell membranes
to current oflow frequency (see Schwan, 1957). Many observations have
been made on excised material and on suspensions ofred cells, in which
the electrical properties of the cell membrane have been established
(Fricke, 1925, 1933). Fewerobservationshavebeenmadeontissuesinsitu.
Peserico (1926) andBronk & Gesell (1926) have measuredtheresistance of
248 D. C. CLARK AND OTHERS
salivary gland, and both the latter workers and Van Harreveld, Potter &
Sloss (1961) observed that the resistance ofthe gland increased whenthe
chorda tympani nerve was stimulated. Ofthe other glandular tissues only
liver (Schwan & Kay, 1956, 1957) and kidney (Lofgren, 1951) appear to
have been studied.
As part of an investigation of the cellular changes produced in the
exocrinetissue ofthe cat'spancreas byhormonal stimulationtheelectrical
properties ofthe gland have been studied, and correlated with alterations
in its chemical composition.
METHODS
Theexperimentswereperformedonfastedcats,anaesthetizedwithchloralose(75mg/kg)
and urethane (37.5mg/kg). The splanchnic nerves were cut extraperitoneally, the pyloric
sphincter occluded with a tape ligature, and the pancreatic duct cannulated. Electrodes
were placed on either side of the tail of the pancreas. In the earlier experiments these
consisted oftwo rectangular platinum plates, but to avoid possible artifacts which might
result from swelling ofthe stimulated pancreas amore elaborate three terminal electrode
system, ofthe 'guardring' type described byGriffiths & Lee (1962), wasused inthelater
experiments. The circular electrodes were mounted on a Perspex frame, and the lower
electrodewasfixed.Thepositionoftheupperelectrode,0-8cmindiameter,couldbealtered
byamicrometer attachmentwhichallowedaccuratemeasurement ofthedistancebetween
theelectrodes. Thelowerelectrodeconsistedofaninnerdisk,0-52cmindiameter,surrounded
byanouterguardring, 0-13cmwide,withaninsulatingzoneofPerspex,lessthan0*1mm
wide, betweenthe two. The guardringwasconnected totheneutralline ofthemeasuring
bridgesandtheupperandlowercentralelectrodeswereconnectedtotheirvoltage (E) and
current (I) terminalsrespectively.
Throughtheseelectrodes alternating currentwasappliedacrossthetailofthepancreas,
and the impedance ofthe gland measured at frequencies between 1kc/s and 5Mc/s. The
currentwas supplied byaMarconibeat-frequency oscillator, forfrequencies upto 30kc/s,
and an Airmec 858 oscillator for higher frequencies up to 5Mc/s. The detectors were a
Tektronix 502 oscilloscopeandanAirmecWaveAnalyser853,andthemeasurementswere
made withtwoWayne KerrbridgesB221 andB601. Theappliedvoltagevariedbetween
05and 35mVr.m.s., andwithinthisrange the impedancewasconstant.
Piecesoftissueremovedfromthetailofthepancreasforchemicalanalysiswereweighed,
dried overnight at 1000 C and reweighed. The fat content ofthe drytissue was measured
bythemethodofHastings &Eichelberger (1937).Theconcentrationsofsodiumandpotas-
sium were determined by flame photometry after digestion ofthe dry tissue in sulphuric
andnitricacids(Flink, Hastings& Lowry, 1950).Thechlorideconcentrationwasmeasured
bythemethod ofSanderson (1952) afterextraction ofthewettissuewithnitric acid. The
secretin used to stimulate the pancreas was prepared by the method ofCrick, Harper &
Raper (1949). Resultsare expressedasmeans +s.E. (no. ofobservations).
RESULTS
The electricalproperties ofthe restingpancreas. Both the resistivity and
dielectric constant ofthe gland varied with the frequency ofthe applied
current. In six experiments the resistivity was 1031 + 107 Q cm and the
dielectric constant 3-17+0x65x 105at 1 kc/s. At 2 Mc/stheyhaddecreased
to 225+ 5 Q cm (9) and 6x2+2x4x 102 (9) respectively (Figs. 1 and 2). The
ELECTRICAL PROPERTIES OF PANCREAS 249
impedancelocusofthepancreaswasobtainedbymeasuringtheimpedance
ofthetissue overarangeoffrequenciesfrom 1 kc/sto 3 Mc/s. Ifthemeans
of the specific reactance in nine experiments were graphed against the
means of the resistivity the points fell on a semicircle with a centre de-
pressed below the resistivity axis. When the curve was extrapolated to
zero frequency the resistivity axis was cut at 986+49 Q cm. The cor-
responding value forinfinite frequencywas 201 +5 Q cm. Thephase angle
betweenlinesfromthecentreofthecircletotheinterceptatzerofrequency
andvertical tothe resistivity axis hadameanvalue of64*9+ 1.50 (Fig. 3).
The characteristic frequency was found to be 98&9+ 12-6 kc/s (Fig. 1).
1200
1000
0 800
0C)
0
ea 600
.0_
0
0 400
CC
200
0 10 100 1000
Frequency (kc/s)
Fig. 1.Therelationshipofspecificresistanceofpancreatictissuetothefrequencyof
the applied alternating current. The curve is drawn through the means of nine
experiments, andthebarsindicate S.E.ofthemeans.Thecharacteristicfrequency
(CF), at which the specific resistance is the mean between the resistance at zero
(ro) andinfinite (r.) frequency, is98-9kc/s.
The fat content ofcat pancreas varies considerably in different animals
(Clark, Harper & Scratcherd, 1961). Schwan (1957) hasfound that fat has
a high specific resistance of 1500-5000 Q cm, andit seemedlikelythatthe
total resistivity of the tissue would be influenced by its fat content. In
fourteen experiments the specific resistance of the pancreas at 1592 c/s
wasmeasured1hraftercompletion oftheoperativeprocedure. Theneutral
fat content ofa sample ofpancreas removed at the end ofthe experiment
250 D. G. CLARK AND OTHERS
was determined. It was found that there was a highly significant increase
(P < 0 002) in the specific resistance ofpancreatic tissue with increase in
its fat content (Fig. 4).
The electrical properties of the secreting pancreas. In the fasted cat,
anaesthetized with chloralose and urethane, there is little or no flow of
pancreatic juice unless the gland is stimulatedby secretin to secrete water
and bicarbonate. Secretin has no direct effect on enzyme secretion,
106 r-
w
C
0 104
0
-.0_C
9z
0 10 100 1000
Frequency (kc/s)
Fig. 2. The dielectric constant ofpancreatic tissue as afunctionoffrequency.
although the first sample of secretin-stimulated juice collected from a
previously quiescent pancreas may contain a considerable amount of
enzymes which has accumulated inthe pancreatic ducts. With continuous
or repeated administration ofsecretin the enzyme content ofthe juice is
very low, although the glandular tissue contains a large amount of
zymogen material (Harper & Vass, 1941; Harper & Mackay, 1948).
Observations were made onthe conductance and capacitance across the
pancreas, secreting in response to single i.v. injections of 0 1-0*5 mg
secretin. Most ofthe measurements were made at a frequency of 1592 c/s.
This frequency, apartfrom its incidental convenience in the calculation of
ELECTRICAL PROPERTIES OF PANCREAS 251
400
200 60
400
200
o
10
cCD)a
C) N\1 .1 kc/s
Cc;2 °
0
200
Specific resistance
Fig. 3. Anexample oftheimpedance locus ofpancreatic tissue. The pointsrelate
the specific reactance and specific resistance over the range offrequencies from
1 to 3000kc/s. The line isthebest-fit circular arc, andthe phase angle is 640.
1200
0
0
1100
0
0 1000
a
0
c;
OQ
, 900
CcC)
C4
800
0
700
0 2 4 6 8 10 12 14 16
Fat content ofpancreas (% byweight)
Fig. 4. Theeffect offat onthe specific resistance ofthe pancreas. The line, fitted
by the method of least squares, has a slope of 25-0 and a y-axis intercept at
762 Elcm, r = 0-776.
252 D. G. CLARK AND OTHERS
conductivity, is sufficiently high to eliminate polarization effects, and low
enoughto avoid anyappreciable currentflowacross themembranes ofthe
pancreatic cells. It seemed likely therefore that electrical changes at this
frequency would reflect alterations in the volume or composition of the
excracellular fluid of the gland. In resting pancreas at this frequency in
twenty-three experiments the range of conductivity was 0-73-1-81 m-
mho/cm. After the injection of secretin capacitance and conductance
showed parallel alterations, but as the capacitive component is only a
16
15
2, 14
C)
13
94
92
- 90
x
0
88
i 86
U 84
82-
80
-
I SN
0 5 10 15 20 25
Minutes
Fig.5.Anexampleoftheeffectsonthecapacitanceandconductance ofthepancreas
oftheintravenous injection of0.5mgsecretion (SN).
small fraction ofthe total impedance at this frequency (Schwan & Kay,
1957) the results will be described in terms of conductance. The rate of
secretionwas determinedbyrecordingtheintervalbetweendrops ofjuice.
Within 10-20 sec ofa secretin injection there was a slight briefincrease
in pancreatic conductance followed by a more marked reduction in con-
ductance. The duration of these phases varied with the dose of secretin,
but on average the first phase of increased conductance was over in
50-100 sec, after which the conductance decreased rapidly to reach a
ELECTRICAL PROPERTIES OF PANCREAS 253
minimum in about 5min, and then slowly returned to the prestimulation
level in 15-20 min (Fig. 5). In nine responses to 0-5 mg secretin in seven
animals the increase in conductance was 0-8+020% and the decrease
6-6+0-5% of the resting conductance level. Secretion usually began
within a minute of the injection, and the rate of flow rapidly reached a
maximum, followed by a gradual decline to zero.
0X3
0-2
0
0 K
2
4
6LL
Fig. 6. Themeasuredrateofflowofpancreatic juice (steppedline) andchangein
conductance, AG, across the pancreas (crosses) after intravenous injection of
0-25mgofsecretinatthedouble-headedarrow.Theshadedareasindicatetherate
offlowand conductance change calculated from equations 1 and2 inthetext.
There is some evidence (seeDiscussion) that the slight increase in con-
ductanceistheresultofatransientincreaseinbloodflowandbloodcontent
ofthepancreasfollowinginjectionofsecretin. Ongraphingtheconductance
changesandrateofflowofpancreatic juiceitbecame clearthatthesecond
phase of decreased conductance followed by a gradual return to resting
level was related to the periods of increasing and declining rates of
secretion (Fig. 6). The secretory response and conductance changes pro-
254 D. G. CLARK AND OTHERS
duced in response to twenty-nine injections of secretin in nine animals
have been examined in detail. The amount of secretin injected (0x125
0 50 mg) produced a submaximal secretory response, in the sense that the
peak rate of flow lasted for a brief time whereas with larger doses the
maximum flow rate persisted for several minutes. It was found that the
rate offlow and the second phase ofthe conductance change in these sub-
maximal responses couldbe characterized by.two closelyrelated empirical
equations.
The rate offlow ofjuice (dv/dt) can be represented by the equation
dv
= K1 In dose e-t/1r (1-e-t/"2) (1)
and the change in conductance (AG) during the phase of reduced con-
ductance and recovery by the equation
AG = K2ln dose e-t'/r (1-et/73), (2)
where K1 and K2 are constants, 'dose' = amount of secretin injected in
/lg, t = the time from the beginning ofsecretion in seconds, and r1,I2 and
T3 are time constants related respectively to the declining phase ofsecre-
tion, the rising phase ofsecretion, andthereduction in conductance. They
have been found to be: 286+ 6 sec (37), T2, 24+7 sec (6) and T3,
l,
220+ 20 sec (7). From equation (1) the halflife ofthe secretory action of
secretin (Ti) can be determined, as it equals 0-6932 r1,i.e. 199+4 sec. The
volume of pancreatic juice secreted in ml. (V) is proportional to the
logarithm ofthe dose ofsecretin injected, a relationship which is defined
by integration ofequation 1 in the form
T2
V = K1ln dose 1 (3)
.
71+r72
In experiments at frequencies other than 1592 c/s it was found that the
general pattern ofthe electrical changes after single injections ofsecretin
was unaltered, but the extent of the reduction in conductance decreased
at higher frequencies. This decrease appeared to bear a linear relationship
to the logarithm of the frequency. The smaller capacitance change also
decreased as the frequency of the applied current was increased until at
the characteristic frequency no change in capacitance could be detected.
Above this frequency there was a slight increase in capacitance (Fig. 7).
In response to a continuous intravenous infusion ofsecretin, provided the
volume oftheinfusionwasinsufficient to alterappreciablythevolumeand
composition of the extracellular fluid, the conductance across the gland
decreased when secretion was established and remained at this level
throughout the period ofinfusion.
ELECTRICAL PROPERTIES OF PANCREAS 255
Electrical characteristics ofcat blood. For comparison with the electrical
measurements onthepancreas, theresistivityoftensampleseach ofwhole
blood and of plasma at 38.50 C was measured. The mean results were
137+2 Q cm for whole blood and 61 +0-65 Q cm for plasma.
The electrolyte composition ofthe resting and secretingpancreas. In eight
animals small portions of glandular tissue were removed from the tail of
the pancreas at rest and during secretion in response to injection of 1 mg
secretin. The second sample wasremoved atthetime, usually about 5 min
-8
66
e;
U
4
-
0~~~~~~~~~~
c;2
0- o
0-
+2
1 10 100 1000
Frequency (kc/s)
Fig.7. Themaximumpercentagechangeinpancreaticconductance U and capaci-
tance * during the second phase of the response to secretin, as a function of
frequency.
TABLE 1
K Na Na+K Cl H20
Restinggland 94-2+2-2 56-1+3-3 150-4+5-0 46-6+0-7 75-1+0-5
Stimulatedgland 82-1+2-7 70-4+3-2 152-5+4-3 48-1+0-9 76-3+0-7
Meanresults +SE. ofeightexperiments. Electrolytes:m-equiv/kgfat-freetissue. Water
percentage offat-free tissue.
after the secretin injection, when a simultaneous conductance record had
reached its lowest value. The sodium, potassium and chloride concentra-
tions in the tissue were determined (Table 1). In all the experiments there
werereciprocalchangesinthepotassiumandsodium concentrationsduring
secretion. The mean resting potassium concentration of 94-2 m-equiv/kg
wet fat-free tissue fell to 82-1 m-equiv/kg on stimulation, and the cor-
responding figures for sodium were 56-1 and 70-4 m-equiv/kg. The fall in
potassium and rise in sodium concentration on stimulation were both
256 D. G. CLARK AND OTHERS
statistically significant (P < 0'01). The slight increases in the concentra-
tions of chloride, water and in the sum of the sodium and potassium
concentrations on stimulation were not significant.
DISCUSSION
The electrical properties ofthe resting pancreas are similar to those of
other tissues which have been investigated. Schwan & Kay (1557) found
thatinliverat 1kc/stherangeofresistivitywasbetween765and1040Qcm
andthedielectricconstant 3-2+0-85 x 105. Thecorrespondingmeanfigures
for the pancreas are 1031 + 107 Q cm and 3-17+ 0X65x 105. The decrease
in pancreatic resistivity and dielectric constant withincreaseinfrequency
ofthe applied current is also in agreement with observations on liver and
other tissues (Schwan, 1957). The similarity is emphasized by considering
theimpedancelocusofthepancreasandothertissues. Theimpedancelocus
of the pancreas is characterized by the phase angle of 64 9+ 1 5', which
compares with 56-63' for frog sciatic nerve (Cole, 1934), 70-78' for frog
gastric mucosa (Teorell & Wersaill, 1945), and figures of 670, 710 and 750
for skeletal muscle (Cole, 1934; Guttman, 1939; Schwan, 1954, respec-
tively).
The observed relationship between impedance oftissues and frequency
ofapplied current is the resultant ofthree groups ofelectrical relaxations,
designateda, /3andy. Theacandyrelaxationsinfluence themeasurements
atlowandhighfrequencies respectively. Most ofthemeasurements onthe
pancreas, which are the basis of the resistivity-frequency relationship
(Fig. 1) and the derived impedance locus (Fig. 3), were made over the
frequency range dominated by the /3 relaxation. At the lower frequencies
thereisevidenceofanacdispersioninthediscrepancybetweentheobserved
relationships of specific reactance and specific resistance at these fre-
quencies and the impedance locus extrapolated from the main ,3 dis-
persion. This accounts for the apparent anomaly of a measured mean
resistivity of 1031 Q cm at 1 kc/s and an estimated 961 Q cm at zero
frequency.
Fromtheresistivity ofpancreatic tissue andtheproportion ofthetissue
occupiedbycells approximate valuesfortheresistivityoftheextracellular
and intracellular fluids may be obtained by applying the following
equations from the theory of electrical resistivity of cell suspensions,
developed by Fricke (1924, 1933) and Velik & Gorin (1940)
r2+(2
r= ) (4)
O)r2
r r (1- )rl+(2+
r-1 (1+25b)rl+2(1- r2'