Effect of neural stimulation on acinar cell membrane potentials in isolated pancreas and salivary gland segments

Effects of adrenergic neurotransmitter on K transport in superfused segments of rat submaxillary gland

The effect of the adrenergic neurotransmitter on K transport in the segments isolated from the rat submaxillary gland was investigated, employing the technique of electrical field stimulation (FS) and applying tyramine, a releasing drug for the catecholaminergic neurotransmitter. FS (16 Hz, 2 ms and 80 V for 1 min) caused a K release (peak value, 1.0 mumol/g/min) followed by a K uptake (peak value, 0.4 mumol/g/min) in the absence of any autonomic antagonist. Both FS-induced K release and uptake were blocked by the addition of tetrodotoxin (10(-7) g/ml). In the presence of atropine (1.4 X 10(-6) M), FS caused only a transient K uptake, which was abolished by the superimposed addition of propranolol (5 X 10(-6) M). Application of tyramine (6 X 10(-6)-6 X 10(-4) M) always only caused K uptake (peak value, 0.96 mumol/g/min), which was abolished by the addition of propranolol. The K uptake evoked either by FS or by tyramine application in the presence of atropine was not seen in the segments from rats pretreated (i.p.) with 6-hydroxydopamine. These results suggest that the adrenergic neurotransmitter activates mainly a beta-adrenergic receptor and evokes K uptake through receptor activation.

Neuroendocrine control of secretion in pancreatic and parotid gland acini and the role of Na+,K+-ATPase activity

The results of our investigations into the localization of Na+,K+-pump activity in pancreatic and parotid acinar cells and the effects of hormones and neurotransmitters on pump turnover can be integrated with data on other aspects of stimulus-response coupling to construct models of the neurohumoral control of protein, fluid, and electrolyte secretion (Fig. 23). In both tissues, Ca2+ and cyclic AMP serve as intracellular messengers. In pancreatic acinar cells, the Ca2+-dependent pathway activated by the occupation of CCK or cholinergic receptors provides the primary stimulus for digestive enzyme secretion. Cyclic AMP plays a comparatively minor role; VIP and secretin are much less effective stimulators of protein secretion. Conversely, cyclic AMP levels in parotid acinar cells, which are modulated primarily through occupation of beta-adrenergic receptors, are a major determinant of enzyme secretion. Activation of the Ca2+-dependent pathway by cholinergic or alpha-adrenergic agonists or substance P is less important. The presence of dual control processes in each gland suggests that the observed differences in effectiveness of cyclic AMP- versus Ca2+-dependent secretagogues may reflect not different mechanisms, but rather a shift in the relative emphasis placed on each pathway. This emphasis could conceivably result from subtle variations in the interaction between cellular protein kinases and phosphatases and their phosphoprotein substrates. Electrolyte secretion, on the other hand, appears to involve both discrete and common entities. In pancreatic acinar cells from rodent species, cholinergic or CCK receptor occupancy elicits a Ca2+-dependent increase in the open-state probability of nonselective cation channels in the basolateral plasma membrane. The resultant influx of Na+ and efflux of K+ is most probably the factor which activates Na+, K+-pumps. Based on electron probe studies of the effects of cholinergic agonists on acinar cell Na+ and K+ contents discussed earlier, a transient reduction in the intracellular K+/Na+ ratio of up to 4-fold may occur. A shift of this magnitude in the cytoplasmic microenvironment of the Na+, K+-pump clearly would have a stimulatory influence (see discussion by Jorgensen, 1980). In addition, Ca2+ itself may have direct effects on Na+,K+-pump activity. Calcium at levels much above 1 microM progressively inhibits Na+,K+-ATPase activity (Tobin et al., 1973; Yingst and Polasek, 1985). In unstimulated guinea pig pancreatic acinar cells, Ca2+i measured by quin-2 fluorescence was 161 +/- 13 nM (Hootman et al., 1985a) which increased to a maximal concentration of 803 +/- 122 nM following CCh stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Beta-adrenergic receptor mechanisms in rat parotid glands: activation by nerve stimulation and 3-isobutyl-1-methylxanthine

The technique of electrical field stimulation (e.f.s.) was employed in conjunction with selective pharmacological antagonists to specifically investigate the role of endogenous neurotransmitter(s) in the activation of beta-adrenergic receptor mechanisms in isolated parotid gland segments of the rat. The field-stimulus-induced amylase release due to beta-adrenergic receptor activation was characterized as that persisting in the presence of atropine (10(-5) M) and phentolamine (10(-5) M) and susceptible to blockade by propranolol (5 X 10(-6) M), i.e. combined beta 1- and beta 2-receptor blockade. The selective beta 1-receptor antagonist metoprolol (10(-5) M) was as effective as propranolol in blocking the beta-mediated enzyme release. The selective beta 2-receptor antagonist, H35/25 (10(-5) M) did not significantly affect the field-stimulus-induced amylase release. In the absence of any phosphodiesterase inhibitor the levels of cyclic AMP in the tissues were close to the limit of detection. Field stimulation was however associated with a fourfold increase in cyclic AMP. By comparison isoprenaline (10(-5) M) gave rise to a tenfold increase in cyclic AMP. The changes in cyclic AMP metabolism, in response to both field stimulation and isoprenaline, were greatly enhanced in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). The field-stimulus-induced increase in cyclic AMP was abolished by the beta 1-adrenergic receptor antagonist, metoprolol, but persisted in the presence of the beta 2-adrenergic antagonist, H35/25. IBMX was found to have a potent direct effect on amylase release. IBMX (10(-3) M) also gave rise to a tenfold increase in cyclic AMP. IBMX is then as effective as 10(-5) M-isoprenaline in stimulating both enzyme secretion and cyclic AMP metabolism. The secretory response to IBMX was unaffected by beta-adrenergic blockade by propranolol, was independent of extracellular calcium and did not give rise to 86Rb+ efflux. Importantly, isoprenaline (10(-5) M) failed to evoke any significant increase in amylase release if introduced during sustained superfusion of IBMX, yet it is in such protocols that the greatest changes in cyclic AMP metabolism are seen. The study clearly demonstrates that the beta-adrenergic-receptor-regulated amylase release in response to nerve stimulation is mediated predominantly, if not exclusively, by the beta 1-receptor subtype.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of nerve stimulation on enzyme secretion from the in vitro rat pancreas and 3H-release after preincubation with catecholamines

In the presence of the cholinergic antagonist atropine, electrical field stimulation (FS) (5-20 Hz) caused a marked, reversible increase in the amylase output from superfused rat pancreatic segments. Adrenaline and noradrenaline evoked dose-dependent increases in amylase output which were similar to those produced by FS. The FS- and catecholamine-evoked amylase secretions were abolished by the beta-adrenergic antagonist propranolol. The FS-evoked secretion could be abolished by either the removal of external Ca2+ or the application of tetrodotoxin (TTX, 2 X 10(-6) M). FS also resulted in a reversible increase in the fractional efflux of tritium (3H) from rat pancreatic tissues preincubated with either 3H-noradrenaline or 3H-adrenaline. The effects of FS (5-20 Hz) on 3H efflux were abolished by TTX (2 X 10(-6) M). TTX had no effect on the enhancement of 3H efflux caused by elevation of external potassium concentration (high K+, 75 mM). Removal of superfusate Ca2+ completely abolished both the FS- and high K+-induced increases in 3H efflux. These observations suggest that intrinsic nerve stimulation (i.e. FS) results in the Ca2+-dependent release of sympathetic neurotransmitter, noradrenaline, which has a direct secretory action on the rat pancreas. Furthermore, the findings suggest that adrenaline can be taken up by nervous elements. This raises the possibility that uptake and re-release of circulating adrenaline might contribute to the control of rat pancreatic enzyme secretion by the adrenergic nervous system.

Nervous control of membrane conductance in mouse lacrimal acinar cells

Intracellular microelectrode recordings were made from superfused in vitro preparations of mouse lacrimal gland. The lacrimal acinar cell had a mean resting membrane potential of -44.1 +/- 0.5 mV and a mean input resistance of 3.5 +/- 0.15 M omega. Electrical field stimulation (FS) had similar effects to ACh applied by microionophoresis, both evoking a biphasic membrane hyperpolarization (up to 15 mV) accompanied by a reduction in input resistance. The equilibrium potential values (EFS and EACh) for the responses to brief duration FS and ACh ionophoresis ranged between -45 and -75 mV and depended on the time at which measurements were made following the onset of stimulation. Superfusion of ACh or adrenaline also caused membrane hyperpolarization and increased membrane conductance. Estimations of EFS and EACh made during prolonged periods of FS and ACh superfusion yielded mean values of -53.9 +/- 1.9 mV and -53.4 +/- 1.5 mV respectively. FS evoked a response in all preparations tested with maximal effects seen at 40 Hz frequency. The mean latency of the FS-evoked hyperpolarization (40 Hz) was 270 +/- 21 ms and that for the ACh ionophoretic response was 400 +/- 65 ms. Low frequency FS (0.5-5 Hz) also induced membrane hyperpolarization and responses to single shock stimuli were occasionally observed. The FS-evoked hyperpolarization was abolished following the blockade of nerve conduction by superfusion of either Na-free or tetrodotoxin-containing media. Effects of FS were not seen in the presence of atropine. Neostigmine potentiated the FS- and ACh-evoked hyperpolarizations.(ABSTRACT TRUNCATED AT 250 WORDS)

Substance P is a functional neurotransmitter in the rat parotid gland

The technique of electrical field stimulation was employed to stimulate the intrinsic nerves of isolated rat parotid gland fragments. Responses to field stimulation were recorded as changes in enzyme secretion (amylase release), radiolabelled ion fluxes (86Rb efflux) and electrophysiological effects (changes in acinar cell membrane potential and input resistance). All effects of field stimulation were abolished by the neurotoxin, tetrodotoxin (TTX). Selective use of pharmacological antagonists revealed that both the sympathetic and parasympathetic nerves to this tissue were being excited by field stimulation. Importantly a significant component of the response to field stimulation persisted in the presence of combined autonomic receptor blockade by atropine, phentolamine and propranolol, i.e. due to release of a non-cholinergic, non-adrenergic neurotransmitter. The non-cholinergic, non-adrenergic neurotransmitter evoked amylase release, 86Rb efflux and electrophysiological effects seen as changes in acinar cell membrane potential and conductance, i.e. stimulus-permeability coupled. Two biologically active peptides, substance P (SP) and vasoactive intestinal polypeptide (VIP) were shown to evoke amylase release in the presence of combined autonomic blockade. VIP however did not evoke any increase in 86Rb efflux, i.e. not stimulus-permeability coupled. All the effects of the non-cholinergic, non-adrenergic transmitter were mimicked by substance P which evokes 86Rb efflux and electrophysiological effects in addition to amylase release. The non-cholinergic, non-adrenergic field stimulus effects on amylase release and 86Rb efflux were abolished or markedly attenuated in tissues which had been desensitized by prior exposure to exogenous substance P. In the presence of VIP, however, the non-cholinergic, non-adrenergic effects persisted and were apparently potentiated. Acute application of the neurotoxin capsaicin first stimulated a transient release of amylase and subsequently abolished the non-cholinergic, non-adrenergic field stimulus-evoked enzyme release. The putative substance P antagonist, D-Pro2, D-Trp7,9 substance P, reversibly blocked the response to both non-cholinergic, non-adrenergic nerve stimulation and exogenous substance P. It was demonstrated however that prolonged exposure to this antagonist is associated with non-reversible and, importantly, non-specific neurotoxic effects. It is concluded that substance P or a closely related peptide is a functional neurotransmitter in the rat parotid gland.

Ionophore A23187 can mimick the changes in membrane permeability that occur during acetylcholine-stimulation of pancreatic acinar secretion

Acetylcholine (ACh) released from vagal terminals increases the permeability of the pancreatic acinar membrane to Na+ and Ca2+ ions. In this report, we compare the induced changes in intracellular Na+ and Ca2+ electrode potentials (ENa and ECa) due to ACh-stimulation of acini with those observed during stimulation with the calcium ionophore, A23187, which mimicks the action of ACh on pancreatic secretion. Stimulation with ACh concentrations varying from 10(-8) to 10(-5) M and with A23187 concentrations of 10(-6) and 10(-5) M caused parallel increases in cytosolic Ca2+ and Na+ ([Ca]i, [Na]i). The magnitude of the increases in [Ca]i and [Na]i due to A23187-stimulation further indicate that when presented with a calcium challenge the acinar cells continue to regulate [Ca]i close to physiological levels and suggest that the observed increases in ionized calcium could reflect much larger increases in complexed Ca2+. ACh-stimulation following removal of either extracellular Na+ or Ca2+ ions, eliminated the intracellular increases found when the removed ions is present, but did not affect the increases usually found with the other ion. The independence of the permeability changes to either the presence of Ca2+ or Na+ indicates the ACh-induced currents carried by Na+ and Ca2+ are also independent. The selective translocation of Na+ and Ca2+ during acetylcholine-stimulation in a manner analogous to the changes observed when ionophore A23187 was used as stimulus, indicates the ability of the activated acinar membrane to function as an ionophore.

Control of enzyme secretion by non-cholinergic, non-adrenergic nerves in guinea pig pancreas

Depolarization of pancreatic cells by exposure to high potassium solutions is associated with release of amylase. In the guinea pig, but not the mouse or cat, this Ca-dependent amylase secretion is resistant to atropine blockade, thus Scheele and Haymovits concluded that the enzyme secretion evoked by K depolarization does not involve release of transmitter from intrapancreatic nerves but is a consequence of Ca uptake into acinar cells mediated by the membrane depolarization. This hypothesis is inconsistent with current concepts of stimulus--secretion coupling in electrically non-excitable cells. The observation of Scheele and Haymovits could, however, also be explained by the release of a non-cholinergic, secretomotor transmitter as a consequence of the depolarization of intrapancreatic nerves. By adapting the technique of electrical field stimulation of isolated pancreatic segments to our studies of amylase secretion, we have now been able to demonstrate both cholinergic and non-cholinergic, non-adrenergic secretomotor nerves in the guinea pig pancreas. Excitation of the non-cholinergic nerves stimulates amylase secretion by a different intracellular coupling mechanism from that activated by cholinergic nerves or by peptides belonging to the cholecystokinin, gastrin or bombesin families.

ACh-evoked complex membrane potential changes in mouse submaxillary gland acini. A study employing channel blockers and atropine

The responses in membrane potential and resistance of acinar cells to iontophoretically applied acetylcholine (ACh) were investigated using intracellular micro-electrode recording in superfused segments of mouse submaxillary gland. For measurements of membrane resistance and acetylcholine equilibrium potential (EACh), two micro-electrodes were inserted into neighbouring communicating cells. Current could be injected through one of the electrodes. The pattern of membrane potential change induced by ACh depended on the resting potential. Simple hyperpolarizations were induced at low resting potentials, while biphasic potential changes (depolarization followed by hyperpolarization) or simple depolarizations were observed at relatively high resting potentials. A similar dependence of the ACh induced potential change on the resting potential was obtained in experiments in which the resting membrane potential was set at different levels by injecting direct current and stimulating the same cell with equal doses of ACh. The ACh equilibrium potential ranged widely between -45 and -75 mV. Under special conditions the conversion in response to ACh from a hyperpolarization to depolarization could be obtained without change in resting potential. Small doses of ACh evoked simple depolarization, while medium doses induced biphasic responses and large doses of ACh caused hyperpolarization. The effect of a low concentration of atropine on the response was an initial block of hyperpolarization followed by a secondary block of depolarization. Intracellular injection of TEA ions converted the ACh induced potential response from hyperpolarization to depolarization. Both the depolarizing and hyperpolarizing ACh responses were accompanied by a marked reduction in membrane resistance. The depolarization was abolished by a severe reduction in external Na concentration, while the hyperpolarization was sensitive to alternations in external K concentration. These results indicate that some of the complex responses in submaxillary gland acinar cells to ACh may be explained by the interaction between two different kinds of potential change (Na dependent depolarization and K dependent hyperpolarization).