Secretin dissipates red aridine orange fluoescence from pancreatic duct epithelium

This study was undertaken to elucidate whether duct cells in the pancreas contain acidic cytoplasmic compartments regulated by secretion. Microdissected pancreatic ducts from pigs were examined by acridibe orange (AO) and 2′, 7′-biscarboxyethyl-5(6)-carboxyfluorescein/tetraacetioxymethyl ester (BCECF/AM) epifluorescence microscopy. Estimated cytoplasmic pH using BCECF fluorescence was 7.43pL0.04 and was not changed by altering CO₂ tension in the incubation mdium. The epithelium of acridine orange incbated peripheral interlobular pancreatic ducts exhibited green and fluorescence was sen in resting pancreatic ducts and was greatly accentuated by raising CO₂ in the incubation medium with chloroqyuine or NH₄Cl or the protonophores carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) or carbonyl cyanide M-chlorophenylhydrazone (CCCP), leaving uniform gren fluoresence. These findings suggest that pancreatic duct cells contain CO₂-dependent acidic compartments which vanishduring seceatin stimulation and which may be cytoplasmic tubulovesicles.     

Comparison of arterial,end-tidal and transcutaneous PCO2 during moderate exercise and external C02 loading in humans

Summary Static relationships between arterial, transcutaneous[/p] and end-tidal PCO₂ (P _aCO₂, P _tc CO ₂, P _etCO₂) as well as the dynamic relationship between P _etCO₂ and P _tcCO₂ were studied during moderate bicycle ergometer exercise with and without external C0₂ loading. The exercise pattern consisted of 5-min intervals of constant power at 40 W and 100 W and 900 s of randomised changes between these two power levels. The external CO₂ loading was achieved by means of controlled variations of inspiratory gas compositions aimed at a constant P _etCO₂ of 6.5 kPa (49 mm Hg). The P_etO2 was regulated at 17.3 kPa (130 mm Hg). Under steady-state conditions all PCO₂ parameters showed close linear relationships. P _aCO₂/P _tcCO₂ was near to identity while the P _etCO₂ systematically overestimated changes in P _aCO₂. No relationship showed a significant influence of the exercise intensity. Transients of P _tcCO₂ are considerably slower than P _etCO₂ transients. The dynamic relationship between both parameters was found to be independent of whether internal or external C0₂ loadings were applied. It is concluded that the combination of P _etCO₂ and P _tcCO₂ measurements allows an improved non-invasive assessment of P _aCO₂. While P _etC0₂ better reflects the transients, P _tcCO₂ can be employed to determine slow changes of the absolute P _aCO₂.     

Interstitial PCO2 and pH in rat hippocampal slices measured by means of a novel fast CO2/H+-sensitive microelectrode based on a PVC-gelled membrane

We describe here the construction and properties of a double-barrelled microelectrode (tip diameter 4–10 m) which permits simultaneous measurements of PCO₂ and pH, and which has a 90% response time of only one or a few seconds for a step change in PCO₂. The fast response of the CO₂-sensitive barrel is due to (i) the use of a PVC-gelled (tridodecylamine-containing) membrane solution which enables the construction of extremely short ( 4 m), yet mechanically stable, membrane columns, and (ii) the presence of carbonic anhydrase in the filling solution. Recordings made in the pyramidal layer of area CA1 in rat hippocampal slices showed that the deviation in the acid direction of the basal interstitial pH (pH₀) from that of the perfusion solution was attributable to a higher PCO₂ level within the tissue. Most of the late acid shift evoked by stimulation of the Schaffer collaterals (5- to 20-s trains at 10 Hz) could also be explained on the basis of an accumulation of interstitial CO₂ at a constant HCO ₃ ^– concentration. This conclusion was supported by the finding that inhibition of extracellular carbonic anhydrase activity by 10 M benzolamide completely abolished the activity-induced fall in pH₀, but not the increase in PCO₂. The initial stimulus-induced alkalosis was accompanied by a slight decrease in PCO₂ only, implying a parallel increase in the interstitial HCO ₃ ^– concentration. Benzolamide produced a dramatic enhancement of the early alkaline shift as well as of the simultaneous fall in PCO₂. The latter effect of the drug unmasks a cellular CO₂ sink that is induced by neuronal activity.     

Influence of hypothermia at different arterial CO2 pressures and local cooling of the spinal cervical cord on the activity of single phrenic motoneurons

Summary In anesthetized guinea pigs the action of acetylcholine, norepinephrine, epinephrine, isoproterenol, and reactive hyperaemia on arterial blood pressure, blood flow in the lower leg (measured by venous occlusion plethysmography), and distribution ofpO₂-values a platinum-O₂-microelectrode was used. By continuous recording of thepO₂ the electrode was moved slowly through the muscle tissue by constant velocity (33 /sec).During the action of norepinephrine, epinephrine, isoproterenol, and following temporary arterial occlusion a linear proportion was found between meanpO₂-values or meanpO₂-gradients and mean blood flow values (between 1.3 and 7.8 ml/min ·100 ml tissue). During the action of acetylcholinepO₂-values decreased in spite of increased blood flow.The reduced number of higherpO₂-values and the decreasedpO₂-gradients appear to indicate a relatively small number of open (perfused) capillaries during the action of acetylcholine. The reduced tissue oxygen delivery is due to enlarged diffusion distances and smaller capillary surface areas.     

In vivo micropunctureP CO 2 measurements

Summary An electrode is presented which permits in vivoP _{CO 2} measurements using a micropuncture technique. The tip diameter of the electrode is only a few microns, the tip is specially designed for measuringP _{CO 2} in small tissue compartements.This work was supported by grants from the Swedish Medical Research Council, The Bergwall Foundation and the Medical Faculty of Uppsala, Sweden.     

Effect of secretin and inhibitors of HCO3 −/H+ transport on the membrane voltage of rat pancreatic duct cells

The aim of the present study was to study the effect of secretin on the electrophysiological response of pancreatic ducts. Furthermore, we investigated the effects of lipid-soluble buffers and inhibitors of HCO₃ ^–/H⁺ transport. Ducts obtained from fresh rat pancreas were perfused in vitro. Secretin depolarized the basolateral membrane voltage, V _bl, by up to 35 mV (n=37); a halfmaximal response was obtained at 3×10^–11 mol/l. In unstimulated ducts a decrease in the luminal Cl^– concentration (120 to 37 mmol/l) had a marginal effect on V _bl, but after maximal secretin stimulation it evoked a 14±2 mV depolarization (n=6), showing that a luminal Cl^– conductance G _Cl- was activated. The depolarizing effect of secretin on V _bl was often preceded by about a 6 mV hyperpolarization, most likely due to an increase in the basolateral G _K+. Perfusion of ducts with DIDS (4,4 — diisothiocyanatostilbene — 2,2 — disulphonic acid, 0.01 mmol/l) or addition of ethoxzolamide (0.1 mmol/l) to the bath medium diminished the effect of secretin. Acetate or pre-treatment of ducts with NH₄ ⁺/NH₃ (10 mmol/l in the bath) depolarized the resting V _bl of –65±2 mV by 16±4 mV (n=7) and 19±3 mV (n=10), respectively. The fractional resistance of the basolateral membrane (FR _bl) doubled, and the depolarizing responses to changes in bath K⁺ concentrations (5 to 20 mmol/l) decreased from 22±1 to 11±2 mV. The Na⁺/H⁺ antiporter blocker EIPA (5-[N-ethyl-N-isopropyl]-amiloride, 0.1 mmol/l) also depolarized V _bl by 10±1 mV, FR_bl increased and the response to K⁺ concentration changes decreased (n=7). Effects of EIPA and ethoxzolamide on V _bl were greater in ducts deprived of exogenous HCO₃ ^–/CO₂. Taken together, the present study shows that secretin increased the basolateral G _K+ and the luminal G _Cl-. The depolarizing effect of secretin was diminished following inhibition of HCO₃ ^– transport (DIDS), or HCO₃ ^–/H⁺ generation (ethoxzolamide). Manoeuvres that presumably led to lowered intracellular pH (NH₄ ⁺/NH₃ removal, acetate, EIPA) decreased the basolateral G _K+. The present data support our previously published model for pancreatic HCO₃ ^– secretion, and indicate that the basolateral membrane possesses a pH-sensitive G _K+.     

Effect of H+ and elevated PCO 2on membrane electrical properties of rat cerebral arteries

Some membrane electrical properties of muscle cells from the middle cerebral artery of the rat were recorded with intracellular microelectrodes. The resting membrane potential (E _m) of this preparation was –63 mV. Reduction of extracellular pH to 7.0 in the face of a constantP _{CO 2}of 40 mm Hg had no significant effect onE _m. Similarly the slope of the steady-state voltage/current curves was not different at pH 7.0 compared to control at pH 7.4. In marked contrast, whenP _{CO 2}was elevated to around 60 to 70 mm Hg there was a rapid hyperpolarization and reduction in the slope of the voltage current curve suggesting an increased conductance for one or more ionic species. In addition elevation ofP _{CO 2}increased the slope of theE _m vs. log[K]₀ curve from 46 mV/decade to 59 m V/decade which is in good agreement with a Nernstian potential for a K⁺ selective membrane. These data suggest that while the smooth muscle cells of rat cerebral arteries are relatively insensitive to a small reduction in extracellular pH; reduction of intracellular pH by elevatingP _{CO 2}induces hyperpolarization by increasing K⁺ conductance (g _k). However, it is not clear from these experiments if theP _{CO 2}effects are mediated entirely by changes in pH or if there is a direct membrane action of CO₂.This work is supported by Grant no. HL27862     

Ventilatory response to oscillating and non-oscillatingPa CO 2 in the anesthetized cat

Summary In 11 adult cats, lightly anesthetized with chloralose-urethane, blood from both common carotid arteries was led into a plastic chamber of 15–20 ml and returned to the carotids at a point 1.5 cm more cranial. By doing so arterial blood was assumed to pool within the chamber and lose itsP _{CO 2} oscillations which are normally known to exist as a result of the respiratory cycle. In control periods blood bypassed the chamber, thus maintaining respiratoryP _{CO 2} oscillations. Spontaneous ventilation was measured spirometrically. The animals were breathing pure O₂.Results. 1. When the sinus (carotid) nerves were intact or sectioned there was no significant difference in ventilation before or after switching from non-oscillating to oscillatingPa _{CO 2}. 2. When the vertebral arteries were ligated a drop in ventilation occurred after turning to oscillatingPa _{CO 2} which was followed by a slight rise above control values after 30–50 sec. This phenomenon was independent of sinus nerve integrity. Thus in hyperoxie condition the smallPa _{CO 2} oscillations known to occur in phase with respiration do not seem to provide a respiratory stimulus to resting ventilation above that generated by the mean level ofPa _{CO 2}. The ventilatory depression after vertebral artery ligation must at this time remain unexplained.     

Properties of the luminal membrane of isolated perfused rat pancreatic ducts

The aim of the present study was to investigate by what transport mechanism does HCO ₃ ^– cross the luminal membrane of pancreatic duct cells, and how do the cells respond to stimulation with dibytyryl cyclic AMP (db-cAMP). For this purpose a newly developed preparation of isolated and perfused intra-and interlobular ducts of rat pancreas was used. Responses of the epithelium to inhibitors and agonists were monitored by electrophysiological techniques. Addition of HCO ₃ ^– /CO₂ to the bath side of nonstimulated ducts depolarized the PD across the basolateral membrane (PD_bl) by about 9mV, as also observed in a previous study [21]. This HCO ₃ ^– effect was abolished by Cl^– channel blockers or SITS infused into the lumen of the duct: i. e. 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB, 10^–5 M) hyperpolarized PD_bl by 8.2±1.6 mV (n=13); 3,5-dichlorodiphenylamine-2-carboxylic acid (DCl-DPC, 10^–5 M) hyperpolarized PD_bl by 10.3±1.7 mV (n=10); and SITS hyperpolarized PD_bl by 7.8±0.9 mV (n=4). Stimulation of the ducts with dbcAMP in the presence of bath HCO ₃ ^– /CO₂ resulted in depolarization of PD_bl, the ductal lumen became more negative and the fractional resistance of the luminal membrane decreased. Together with forskolin (10^–6 M), db-cAMP (10^–4 M) caused a fast depolarization of PD_bl by 33.8±2.5 mV (n=6). When db-cAMP (5×10^–4 M) was given alone in the presence of bath HCO ₃ ^– /CO₂, PD_bl depolarized by 25.3±4.2 mV (n=10). In the absence of exogenous HCO ₃ ^– , db-cAMP also depolarized PD_bl by 24.7±3.0 mV (n=10). The present data suggest that in the luminal membrane of pancreatic duct cells there is a Cl^– conductance in parallel with a Cl^–/HCO ₃ ^– antiport. Dibutyryl cyclic AMP increases the Cl^– conductance of the luminal membrane. Taking together our present results, and the recent data obtained for the basolateral membrane [21], a tentative model for pancreatic HCO ₃ ^– transport is proposed.Parts of this study have been presented at the Scandinavian Physiological Society Meeting in Copenhagen 1986 and 64th Meeting of the German Physiological Society in Homburg/Saar     

Effects of carbon dioxide on extracellular potassium accumulation and volume in isolated frog spinal cord

A 6–10-fold increase inpCO₂ in the superfusing Ringer solution increased the volume of the extracellular space (ECS) and changed the spatial distribution and amplitude of the extracellular K⁺ accumulation which resulted from dorsal root stimulation. Using the increase in tetraethylammonium concentration ([TEA⁺]) resulting from iontophoretic injection of that ion in the extracellular fluid as an indication of the volume of the ECS, it was found that in highpCO₂ the ECS volume in spinal dorsal horn increased by more than 60%. In addition, in the presence of raisedpCO₂ we also observed the following: (1) The rate of diffusion of TEA⁺ into the dorsal horn increased. (2) The accumulation of K⁺ ovoked by single or tetanic stimulation of the dorsal root was less. (3) The clearance of K⁺ was slowed down. (4) The regions where K⁺ accumulated were more restricted. (5) The K⁺ evoked depolarization of the primary afferent fibres decreased. (6) In contrast to TEA⁺, the rate of diffusion of K⁺ into the dorsal horn decreased. The effects of an increase inpCO₂ on K⁺ accumulation and clearance appear to result from an increase in ECS volume and a possible decrease in glial electrical coupling which interferes with glial spatial buffering of K⁺.     

The role of fitness on VO2 and VCO2 kinetics in response to proportional step increases in work rate

Summary The purpose of this study was to determine the effect of fitness and work level on the O₂ uptake and CO₂ output kinetics when the increase in work rate step is adjusted to the subject's maximum work capacity. Nine normal male subjects performed progressive incremental cycle ergometer exercise tests in 3-min steps to their maximum tolerance. The work rate step size was selected so that the symptom-limited maximum work rate would be reached in four steps at 12 min in all subjects. Oxygen consumption (VCO₂) and carbon dioxide production VCO₂ were calculated breath by breath. For the group, the time (mean, SEM) to reach 75% of the 3-min response (T _0.75) for VO₂ increased significantly (P<0.01) at progressively higher work rate steps, being 53.3 (5.5) s, 63.5 (4.6) s, 79.5 (5.0) s, and 94.5 (5.8) s, respectively. In contrast, T _0.75 for VCO₂ did not change significantly [74.9 (7.4) s,. 75.6 (5.0) s, 85.1 (5.3) s, and 89.4 (6.3) s, respectively]. VCO₂ kinetics were slower than VO₂ kinetics at the low fractions of the subjects' work capacities but were the same of faster at the high fractions because of the slowing of VO₂ kinetics. The first step showed the fastest rise in VO₂. While VO₂ kinetics slowed at each step, they were faster at each fraction of the work capacity in the fitter subjects. The step pattern in VO₂ disappeared at high work rates for the less fit subjects. The heart rate response paralleled that of VO₂. We conclude that VO₂ and VCO₂ kinetics are slower in the less fit subjects but only VO₂ kinetics are significantly attenuated in response to proportional step increases in work rate.     

CO2 responsivity in the mouse measured by rebreathing

We have modified the rebreathing method to study CO₂ responsivity in very small mammals. Tidal volume (V _T) and frequency (f) of pentobarbital-anesthetized mice were measured during rebreathing from a closed circuit, primed with 95% O₂, 5% CO₂, through which the gas was constantly circulated at 0.5 l·min^–1. The circuit consisted of T-tube from a plethysmograph, Tygon tubing with compliant element, CO₂ analyzer and pump, in series. CircuitPCO₂ (PctCO₂), which was recorded continuously during spontaneous breathing, rapidly equilibrated with end-tidalPCO₂. CO₂ response curves were constructed from extrapolated minute ventilation ( ),V _T,f and parameters of breath-to-breath timing, respectively, onPctCO₂. Analyses of slopes of the response curves, change from onset of rebreathing to peak response, andPctCO₂ at which the response peaked revealed that CO₂ stimulates by increasingf andV _T and that this is effected by facilitation of central inspiratory-expiratory phase switching and inspiratory drive mechanisms. However, the stimulatory effect of CO₂ on phase switching was not sustained, with maximal effect occurring before peak . The advantages and facility of the modified rebreathing method make it suitable for studies of other small mammals, including neonates.     

APCO2 surface electrode working on the principle of electrical conductivity

APCO₂ electrode working on the principle of electrical conductivity is described. The calibration curve can be linearized according to the formula . This linearity has been tested in thePCO₂ range of 0.93–9.33 kPa (7–70 Torr). For the experiments electrodes are used which have conductivity values of about 50 nS and drifts of maximally 5%/h at aPCO₂ of 5.33 kPa (40 Torr). The response time (T ₉₀) is about 20 s. The temperature sensitivity is 2.4 nS/1 K between 298K–310K. The standard error of the measurements is =0.33 nS. With these electrodes tissuePCO₂ can be measured on the surface of various organs.     

Does the threshold of transcutaneous partial pressure of carbon dioxide represent the respiratory compensation point or anaerobic threshold?

On reaching the respiratory compensation point (RCP) during rapidly increasing incremental exercise, the ratio of minute ventilation (V_E) to CO₂ output (VCO₂) rises, which coincides with changes of arterial partial pressure of carbon dioxide (P _aCO₂). Since P _aCO₂ changes can be monitored by transcutaneous partial pressure of carbon dioxide (PCO_2,tc) RCP may be estimated by PCO_2,tc measurement. Few available studies, however, have dealt with comparisons between PCO_2,tc threshold (T _AT) and lactic, ventilatory or gas exchange threshold (V _AT), and the results have been conflicting. This study was designed to examine whether this threshold represents RCP rather than V _AT. A group of 11 male athletes performed incremental excercise (25 W · min^–1) on a cycle ergometer. The PCO_2,tc at (44°C) was continuously measured. Gas exchange was computed breath-by-breath, and hyperaemized capillary blood for lactate concentration ([la^–]_b) and P _aCO₂ measurements was sampled each 2 min. The T _AT was determined at the deflection point of PCO_2,tc curve where PCO_2,tc began to decrease continuously. The V _AT and RCP were evaluated with VCO₂ compared with oxygen uptake (VO₂) and V_E compared with the VCO₂ method, respectively. The PCO_2,tc correlated with P _aCO₂ and end-tidal PCO₂. At T _AT, power output [P, 294 (SD 40) W], VO₂ [4.18 (SD 0.57)l · min^–1] and [la^–] [4.40 (SD 0.64) mmol · l^–1] were significantly higher than those at V _AT[P 242 (SD 26) W, VO₂ 3.56 (SD 0.53) l · min^–1 and [la^–]_b 3.52 (SD 0.75), mmol · l^–1 respectively], but close to those at RCP [P 289 (SD 37) W; VO₂ 3.97 (SD 0.43) l · min^– and [la^–]_b 4.19 (SD 0.62) mmol · l^–1, respectively]. Accordingly, linear correlation and regression analyses showed that P, VO₂ and [la^–]_b at T _AT were closer to those at RCP than at V _AT. In conclusion, the T _AT reflected the RCP rather than V _AT during rapidly increasing incremental exercise.     

Der Einfluß desP A CO 2 auf die Wirkung der Sinusnervenreizung bei intakten und ausgeschalteten Nn. vagi

Summary In rabbits rebreathing oxygen one carotid sinus nerve was stimulated repeatedly by electrical stimuli of constant intensity, both before and after inactivation of vagi by cold blocking or sectioning. In animals with intact vagi the reflex hyperpnea elicited by the nerve stimulation decreased only slightly with increasing hypercapnia. After inactivation of the vagi the stimulatory effect was, at normalP _{A CO 2}, mostly greater than in the intact condition. But with increasing hypercapnia the decrease of the reflex hyperpnea was generally steeper than in the intact preparation, sometimes reaching even lower values than before inactivation. The depressant effects of the stimulation on systemic blood pressure and heart rate were not distinctly changed by the increase ofP _{A CO 2}, neither with intact nor with inactivated vagi. It is concluded that the ventilatory effect of chemoreceptor afferents is modified both by centralP _{CO 2} and by vagal afferents.

Mit Unterstützung der Deutschen Forschungsgemeinschaft (Wi 165).     

Cellular mechanism of force development in cat middle cerebral artery by reducedPCO2

These studies were undertaken to determine the effect of reducing aPCO₂ below physiological levels on cat middle cerebral artery. Upon reduction ofPCO₂ from 37 to 14 torr (pH 7.4) we observed membrane depolarization and force development. ReducingPCO₂ decreased the slope of theE _m vs. log [K]_o curve and increased the slope of the steady-state I/V relationship suggesting that the change inE _m was due to reduction of outward K⁺ conductance (g _k). Elevation of pH from 7.37 to 7.6 had a very similar effect on these cerebral arterial muscle cells, depolarizing the muscle membrane (reducing theE _m vs. log [K]_o curve) and increasing the slope of the I/V relationship to statistically equivalent values as reduction ofPCO₂. ReturningPCO₂ from 14 to 37 torr rapidly relaxed these preparations, but only transiently. This relaxation was followed by a rebound contraction within 3 min, demonstrating a transient nature for the action of elevatingPCO₂ in cerebral arteries. The response to changing pH_o followed a slower time course but did not change with time. These studies demonstrate that both elevated pH_o and reducedPCO₂ activate cerebral arterial muscle by a mechanism which includes reduction ing _k. However, it can not be determined if these similar responses and reduction, ofg _k are mediated by changing pH_i or mediated through different mechanisms. It is possible that pH_o andPCO₂ can modify cerebral arterial tone by direct mechanisms and not necesarily by their effect on pH_i. It is clear, however, that reduction ofPCO₂ and elevation of pH_o both activate cerebral arterial muscle by a mechanism which includes reduction ofg _k.This study was supported by NIH grant no. HL-32871. Dr. Harder is an established investigator of the American Heart Association     

Analysis of alveolar PCO 2 control during the menstrual cycle

We attempted to analyze how is regulated during progesterone-induced hyperventilation in the luteal phase. A model for the CO₂ control loop was constructed, in which the function of the CO₂ exchange system was described as and that of the CO₂ sensing system as . Using this model, we estimated (1) the primary increase in produced by progesterone stimulation and (2) the effectiveness (E) of the loop to regulateP _{A CO 2}, defined as P _{A CO 2} (op)/P _{A CO 2} (cl) in which op signifies open-loop and cl, closed-loop. These respiratory variables were investigated throughout the menstrual cycle in 8 healthy women. During the luteal phase, on average, increased by 9.4% andP _{A CO 2},B andH decreased by 0.33 kPa (2.5 mm Hg), 0.47 kPa (3.5 mm Hg) and 13.6%, respectively, whileS and did not change significantly. (op) increased progressively on successive days of the luteal phase whileE remained unchanged at a value of 7.9, thus there was a progressive decrease inP _{A CO 2}. The decrease inH was considered to lessen P _{A CO 2} (op) and so reduce the final deviation ofP _{A CO 2} (P _{A CO 2} (cl)) during the luteal phase. The decrease inB was found to be dependent on (op).     

Some aspects of determination of the cardiac output by the rebreathing method

It is shown that the rebreathing method can be used to study the cardiac output even when the ventilation does not correlate with the pulmonary blood flow. pCO₂ was measured in mixed venous blood by equilibrating it with pCO₂ in the alveolar air by the rebreathing method. A gas mixture with a CO₂ concentration close to that in venous blood was produced by the subject himself during rebreathing into a bag with a capacity of 2–3 liters, filled with oxygen. Irregularity of distribution of ventilation relative to blood flow was judged from the shape of the CO₂ concentration versus time curve. If signs of irregularity are present, it is impossible to make p_etCO₂ equal to p_ACO₂ and for that reason pCO₂ of arterialized blood was determined. By means of this correction it is possible to determine the cardiac output of patients with cardiac and pulmonary diseases. The possibility of using standard nomograms for calculating the CO₂ concentration in the arterial and venous blood in the presence of appreciable disturbances of the acid-base balance is discussed.Department of Experimental and Clinical Physiology, A. V. Vishnevskii Institute of Surgery, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician V. N. Chernigovskii). Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 83, No. 2, pp. 245–247, February, 1977.     

Effect of bicarbonate on potassium conductance of isolated perfused rat pancreatic ducts

The aim of this study was to investigate the role of the K⁺ conductance in unstimulated and stimulated pancreatic ducts and to see how it is affected by provision of exogenous HCO ₃ ^– /CO₂. For this purpose we have applied electrophysiological techniques to perfused pancreatic ducts, which were dissected from rat pancreas. The basolateral membrane potential PD_bl of unstimulated duct cells was between –60mV and –70mV, and the cells had a relatively large K⁺ conductance in the basolateral membrane as demonstrated by (a) 20–22 mV depolarization of PD_bl in response to increase in bath K⁺ concentration from 5 mmol/l to 20mmol/l and (b) the effect of a K⁺ channel blocker, Ba²⁺ (5 mmol/l), which depolarized PD_bl by 30–40mV. These effects on unstimulated ducts were relatively independent of bath HCO ₃ ^– /CO₂. The luminal membrane seemed to have no significant K⁺ conductance. Upon stimulation with secretin or dibutyryl cyclic AMP, PD_bl depolarized to about –35 mV in the presence of HCO ₃ ^– /CO₂. Notably, the K⁺ conductance in the stimulated ducts was now only apparent in the presence of exogenous HCO ₃ ^– /CO₂ in the bath solutions. Upon addition of Ba₂₊, PD_bl depolarized by 13±1 mV (n=7), the fractional resistance of the basolateral membrane, FR_bl increased from 0.66 to 0.78 (n=6), the specific transepithelial resistance, R _te, increased from 52±13 cm² to 59±15 cm² (n=11), and the whole-cell input resistance, R _c, measured with double-barrelled electrodes, increased from 20 M to 26 M (n=3). These results are consistent with Ba²⁺ inhibition of the K⁺ conductance. Following removal of exogenous HCO ₃ ^– /CO₂ in the same ducts, stimulation led to a larger depolarization on PD_bl to about –25 mV, and Ba²⁺ had a smaller effect on PD_bl and no significant effect on the resistances. The individual resistances in the duct epithelium were estimated from equivalent circuit analysis. The luminal membrane resistance, R ₁ decreased from about 2000 cm² to 80 cm² upon stimulation. The basolateral membrane resistance, R _bl, remained at 90–120 cm², and the paracellular shunt resistance, R _s, at 50–80 cm². Ba²⁺ increased R _bl of stimulated ducts to about 200 cm², an effect present only if the ducts were provided with exogenous HCO ₃ ^– /CO₂. Taken together, the present results indicate that the basolateral K⁺ conductance of pancreatic ducts is sensitive to exogenous HCO ₃ ^– /CO₂, i.e. without HCO ₃ ^– /CO₂ the conductance becomes very low although the ducts are undergoing stimulation.A preliminary report of the present study has been presented at the XXXI International Congress of Physiological Sciences, Helsiniki, Finland, July 1989     

Chemosensitivity of sympathoexcitatory neurones in the rostroventrolateral medulla of the cat

The hypothesis that sympathoexcitatory neurones within the rostroventrolateral medulla (RVLM) may be chemosensitive was tested in chloralose-anaesthetized cats by artificial perfusion of the RVLM via the left vertebral artery. The baroreceptors and peripheral chemoreceptors were denervated by bilaterally dissecting the carotid sinus and vagus nerves. Either white ramus T₃ (WR-T₃) or the renal nerve was recorded to monitor sympathetic activity. Perfusion with saline or Ringer solution bubbled with CO₂ (10%–100%) produced a rapid and pronounced increase in sympathetic activity and blood pressure. Solutions adjusted to the same pH (pH 5.2 for 100% CO₂) with HCl resulted in a much weaker excitation. A linear relationship between PCO₂ and sympathetic activity was demonstrated. During prolonged perfusion (90 s) sympathetic activity returned to the control level after initial excitation and fell below control levels when perfusion ceased. The sympathetic activity response to CO₂-bubbled solutions was unaffected by blockade of synaptic input by microinjection of CoCl₂ into the RVLM, whereas spontaneous sympathetic activity and the supraspinal somato-sympathetic reflex from intercostal nerve T₄ to WR-T₃ were markedly reduced. It is therefore concluded that sympathoexcitatory bulbospinal neurones in the RVLM are directly chemosensitive to changes in arterial PCO₂ and pH.This paper is dedicated to Prof. Dr. H. P. Koepchen on the occasion of his 65th birthday