Membrane effects mediated by alpha-and beta-adrenoceptors in mouse parotid acinar cells

Cl- secretion by cultured shark rectal gland cells. III. Ca2+ regulation of apical membrane Cl- conductance

Calcium ionophores (ionomycin and A-23187) were employed to assess the effects of increased intracellular Ca2+ concentration ([Ca2+]i) on apical membrane Cl- conductance (GaCl) and rate of transepithelial Cl- secretion in cultured shark rectal gland (SRG) cells. Apical 2 microM ionomycin induced dramatic changes in cellular electrophysiological properties: the apical membrane electrical potential difference (V(a)) depolarized from -66 mV to -46 mV, the fractional resistance of the apical membrane (fRa) decreased from 0.88 to 0.23, and the transepithelial electrical potential difference (Vab) increased slightly from +1.2 mV to +1.4 mV. These effects result from increased GaCl because apical low-Cl- shark Ringer (SR) depolarized V(a) by 32 mV and increased fRa from 0.23 to 0.36. Ionomycin-stimulated Vab or short-circuit current (Isc) results largely from increased Cl- secretion because approximately 80% of the increase in Isc is Cl- dependent. Establishing the Ca2+ dependence of ionophore activation of GaCl was confounded because apical low-Ca2+ SR [Ca2+ concentration ([Ca2+]) 1 mM to 0.1 microM] alone activated this conductive pathway. To establish the Ca2+ dependence of ionophore action, we assessed the effect of ionomycin on Isc in low-Ca2+ SR ([Ca2+] = 0.1 microM) and in SR. In low-Ca2+ SR, apical ionomycin stimulated Isc by 14.0 microA/cm2. In SR (normal [Ca2+]), ionomycin increased Isc further by 27.0 microA/cm2. Superfusing the basolateral surface with 2 microM ionomycin for 8-16 min failed to activate GaCl. In every case, subsequent superfusion of the apical surface with ionophore for 1.5-2 min activated GaCl. Bilateral 4 microM indomethacin (45-min superfusion) failed to block the ionomycin-induced GaCl.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological localization of an agonist-sensitive pool of Ca2+ in parotid acinar cells

Muscarinic stimulation of fluid secretion by mammalian salivary acinar cells is associated with a rise in the level of intracellular free calcium ([Ca2+]i) and activation of a calcium-sensitive potassium (K+) conductance in the basolateral membrane. To test in the intact cell whether the rise of [Ca2+]i precedes activation of the K+ conductance (as expected if Ca2+ is the intracellular messenger mediating this response), [Ca2+]i and membrane voltage were measured simultaneously in carbachol-stimulated rat parotid acinar cells by using fura-2 and an intracellular microelectrode. Unexpectedly, the cells hyperpolarize (indicating activation of the K+ conductance) before fura-2 detectable [Ca2+]i begins to rise. This occurs even in Ca2+-depleted medium where intracellular stores are the only source of mobilized Ca2+. Nevertheless, when the increase in [Ca2+]i was eliminated by loading cells with the Ca2+ chelator bis(2-amino-5-methylphenoxy)ethane-N,N,N',N'-tetraacetate (Me2BAPTA) and stimulating in Ca2+-depleted medium, membrane hyperpolarization was also eliminated, indicating that a rise of [Ca2+] is required for the agonist-induced voltage response. Stimulation of Me2BAPTA-loaded cells in Ca2+-containing medium dramatically accentuates the temporal dissociation between the activation of the K+ conductance and the rise of [Ca2+]i. The data are consistent with the hypothesis that muscarinic stimulation results in a rapid localized increase in [Ca2+]i at the acinar basolateral membrane followed by a somewhat delayed increase in total [Ca2+]i. The localized increase cannot be detected by fura-2 but is sufficient to open the Ca2+-sensitive K+ channels located in the basolateral membrane. We concluded that a receptor-mobilized intracellular store of Ca2+ is localized at or near the basolateral membrane.

The control of ion channels and pumps in exocrine acinar cells

The historical development of ideas concerning mechanisms of exocrine fluid secretion will be traced from the original finding of stimulant-evoked K+ release in 1956 to current models involving Ca2+-activated K+ and Cl- channels.

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)

Dopamine increases potassium efflux in the rat parotid gland by stimulating noradrenaline release from sympathetic nerve endings

By studying 86Rb+ efflux (tracer for potassium) and labelled NA ([3H]NA) efflux from preloaded parotid glands in vitro it is suggested that dopamine stimulates potassium efflux through an indirect sympathomimetic activity--i.e. releasing noradrenaline from sympathetic nerve endings.

Activation by calcium of membrane channels for potassium in exocrine gland cells

In the rat parotid salivary gland, fluid secretion is regulated by alterations in fluxes of monovalent ions. In vitro, stimulation of muscarinic, alpha-adrenergic or substance P receptors provokes a biphasic increase in membrane permeability to K+ which can be conveniently assayed as efflux of 86Rb. The increased 86Rb flux is thought to arise in response to a receptor mediated elevation in [Ca2+]i which activates Ca2+-activated K+-channels. The biphasic nature of the response is presumably due to a biphasic mode of Ca2+ mobilization by secretagogues; a transient response reflects release of a finite pool of Ca from an intracellular store while a more sustained phase results from Ca entry through receptor operated Ca channels or gates. Calcium also mediates an increased Na+ entry which in turn activates the Na+, K+-pump. The mechanism involved in the regulation of monovalent ion channels by Ca2+ is not understood.

Activation of potassium transport induced by secretagogues in superfused submaxillary gland segments of rat and mouse

In order to investigate the actions of acetylcholine (ACh), catecholamines and substance P on K transport in the submaxillary gland, measurements of net K flux to and from the gland tissue using flame photometry, Na efflux from the tissue using radioactive 22Na, and membrane potential and input resistance using micro-electrodes were carried out on isolated superfused segments of rat and mouse submaxillary glands. ACh (5.5 X 10(-8) to 5.5 X 10(-4) M), phenylephrine (5 X 10(-7) to 5 X 10(-4) M) or substance P (10(-9) to 10(-5) M) stimulation for 5 min induced a transient K release followed by a small K uptake after the cessation of stimulation. The K release was markedly enhanced by the simultaneous addition of ouabain (10(-3) M). On the other hand, isoprenaline (2.5 X 10(-9) to 2.5 X 10(-5) M) induced a transient K uptake without any preceding K release. The K uptake was completely blocked by the addition of ouabain. Noradrenaline induced only K uptake at a low concentration (3 X 10(-7) M), but induced transient K release followed by marked K uptake at higher concentrations (3 X 10(-6) to 3 X 10(-4) M). The K release induced by noradrenaline was suppressed by the addition of phentolamine (10(-5) M), while the K uptake was suppressed by propranolol (5 X 10(-6) M). The K release induced by ACh, phenylephrine, noradrenaline or substance P was severely reduced by Ca omission from the superfusing solution and restored by the re-admission of Ca. The isoprenaline- or noradrenaline-induced K uptake was, however, little affected by Ca omission. Application of isoprenaline (2.5 X 10(-6) M) induced an increase in 22 Na efflux. The increase in 22Na efflux was completely abolished in the presence of ouabain. Local application to the tissue bath of isoprenaline (4.7 X 10(-13) to 4.7 X 10(-12) mole) or noradrenaline (5.7 X 10(-12) to 5.7 X 10(-11) mole) in the presence of phentolamine (10(-5) M) induced membrane hyperpolarization without any appreciable change in input resistance. The hyperpolarization was abolished in the presence of ouabain (10(-3) M) or propranolol (5 X 10(-6) M) or in a K-free or low Na solution. Higher doses of both agonists, however, induced depolarization or biphasic responses (initial depolarization followed by hyperpolarization). The depolarizations were accompanied by a moderate reduction in input resistance. It is concluded that in the rat and mouse submaxillary gland acinar cells cholinergic, alpha-adrenergic or substance P stimulation causes K release (and perhaps Na uptake) resulting in activation of the Na-K pump, while beta-adrenergic receptor stimulation might directly activate the Na-K pump resulting in K uptake, or might cause Na uptake resulting in activation of the Na-K pump.

Changes in cytosolic calcium during cholinergic and adrenergic stimulation of the parotid salivary gland

Ca-selective microelectrodes were used to examine calcium transport during acetylcholine (ACh) and Epinephrine (Ep) stimulation of amylase secretion in the parotid gland. The cytosolic concentration of free ionized Ca2+ ( [Ca]i) determined in unstimulated cells was 0.44 +/- 0.04 microM. By measuring the induced changes in intracellular electrode potentials (ECa, EM) we were able to demonstrate that ACh at 10(-9), 10(-8), 10(-7), 10(-6), and 10(-5) M increased [Ca]i by 0.20 +/- 0.02, 0.61 +/- 0.04, 0.53 +/- 0.02, 0.30 +/- 0.05, and 0.14 +/- 0.03 microM. Similarly, Ep increased [Ca]i by 0.14 +/- 0.01, 0.42 +/- 0.06, 0.31 +/- 0.04, 0.15 +/- 0.03, and 0.05 +/- 0.04, respectively. Removal of extracellular Ca2+ significantly (P less than 0.001) altered the changes in ECa in response to ACh and Ep stimulation, thereby demonstrating that the induced increases in [Ca]i must be due to a transmembrane movement of Ca2+. Enzyme secretion was found to vary with the concentration of the stimulus used. Maximal secretion occurred during stimulation using 10(-7) M and 10(-8) M Ep with a suppression of release at supramaximal concentrations. The dose-response curve for ACh differed in that there were two concentrations of stimulus (2 X 10(-9) and 1 X 10(-6) M ACh) in which the greatest rate of secretion occurred. Concentrations of stimulus which increase [Ca]i between 0.86 +/- 0.06 microM and 0.74 +/- 0.05 appeared to produce optimal amylase secretion, indicating that salivary secretion in the mouse parotid is regulated within a narrow concentration range of cytosolic Ca2+.

Nature of the receptor-regulated calcium pool in the rat parotid gland

1. The transient release of 86Rb from parotid slices induced by secretagogues was investigated. 2. In the absence of external Ca, only one transient response (to carbachol) could be obtained. 3. After blocking the cholinergic stimulus with atropine, a second response (to substance P) could be elicited if the slices were briefly (2 min) exposed to a Ca-containing medium. 4. The magnitude of the substance P response depended on the concentration of Ca to which the slices had been exposed. 5. An exposure to Ca of 2 min duration was found to be sufficient to restore maximal substance P responsiveness. 6. These results are interpreted to mean that the cholinergic stimulus elicited a transient 86Rb efflux response by first releasing a finite pool of cellular Ca which could be reloaded from the extracellular space by a brief (2 min) incubation in a Ca-containing medium. The magnitude of the subsequent response to substance P apparently reflects the quantity of Ca taken up by the pool. 7. A number of cationic substances antagonized the restoration by Ca of the substance P response; the rank order of potency was: La3+ = Tm3+ greater than Co2+ = Ni2+ greater than neomycin much greater than Mg2+. 8. These same substances were examined for their relative abilities to inhibit Ca binding to phosphatidylinositol-4, 5-bisphosphate; in this case the rank order of potency was: La3+ = Tm3+ greater than neomycin greater than Co2+ greater than Ni2+ = Mg2+. 9. It is concluded that the uptake process does not appear to reflect Ca binding to phosphatidylinositol-4, 5-bisphosphate.

An electrophoretic study of proteins secreted by the rat submandibular gland in response to autonomic agonists

Proteins secreted by the rat submandibular gland after administration of autonomic agonists have been fractionated by sodium dodecyl sulphate (SDS) - polyacrylamide gel electrophoresis. alpha-Adrenoceptor, beta-adrenoceptor and muscarinic agonists were all found to cause the secretion of different protein populations.

Parotid acinar cells: ionic dependence of isoprenaline-evoked membrane potential changes

The effect of microionophoretic application of isoprenaline on membrane potential and resistance of mouse parotid acinar cells was investigated. For measurements of membrane resistance and the isoprenaline equilibrium potential (Eiso), two microelectrodes were inserted into neighbouring communicating cells. Passing direct current through one of these electrodes, the resting potential could be set at desired levels and Eiso was determined by plotting the relation between the size of the isoprenaline-evoked potential change and the resting potential. Simple depolarizations were found at relatively high resting potentials, while biphasic potential changes in response to isoprenaline (hyperpolarization followed by depolarization) were observed at low resting potentials. Both depolarizing and hyperpolarizing responses to isoprenaline were accompanied by a reduction of membrane resistance. The isoprenaline equilibrium potential in the initial phase of the response was about -53 mV, but had a value of about -24mV in the delayed phase. The initial isoprenaline-evoked potential change was sensitive to alterations in extracellular Na, K and Cl concentrations. The delayed depolarizing response to isoprenaline was markedly reduced by replacing extracellular Na by Tris or extracellular Cl by SO4. These results indicate that isoprenaline opens up conductance pathways permeable to Na and K.

Secretory potentials in the submaxillary gland of the cat

Sympathetic nerve stimulation caused biphasic secretory potentials in cat submaxillary gland cells. The initial phase was a transient hyperpolarization. It was seldom seen after single shocks but nearly always occurred at high stimulation frequencies (10-20 Hz). It was imitated by phenylephrine and completely eliminated by dihydroergotamine or phentolamine and thus considered to be mediated via alpha-adrenoceptors. The late phase, a depolarization, was seen even at low stimulation frequencies and could often be evoked by single shocks. Both alpha- and beta-block reduced this response. Parasympathetic nerve stimulation evoked transient hyperpolarizations. The secretory potentials mediated via all the three receptors classes were paralleled by decreased input resistances across the acinar cell membranes.

The effects of epinephrine, norepinephrine, and phenylephrine on the types of proteins secreted by rat salivary glands

The types of proteins secreted by rat submandibular glands in response to secretory stimulation with epinephrine, norepinephrine, or phenylephrine were dose-dependent. At relatively high doses the proteins secreted were characteristic of alpha-adrenergic stimulation, whereas at low doses they were characteristic of beta-adrenergic stimulation.

A calcium-readmission response recorded from Nauphoeta salivary gland acinar cells

1. When cockroach salivary glands are exposed to bathing solutions without added calcium, reintroduction of calcium causes the acinar cells to hyperpolarize. The effect ('readmission response') may be very prolonged if the conditioning solution contains 5 mM-cobalt. 2. Evidence is presented against the possibility that the readmission response is mediated by transmitter released from the salivary nerves. 3. The readmission response is shown to reflect an increase in potassium conductance.

On the role of calcium in the electrical responses of cockroach salivary gland cells to dopamine

1. The calcium dependence of the intracellularly recorded hyperpolarizing responses of salivary gland acinar cells of Nauphoeta cinerea (Olivier) to ionophoretically applied dopamine has been examined. The results of withdrawing calcium from the bathing solution were essentially the same whether or not other divalent cations were present. The effects of calcium withdrawal were rapidly reversed on replacement of calcium. 2. Small responses to dopamine were reduced or abolished by calcium withdrawal but could be restored by an increase in the amount of ejected dopamine. Calcium withdrawal did not have any consistent effect on the input resistance: a reduction in input resistance is therefore not the main cause of the reduction in the amplitude of the responses to dopamine. 3. In very low calcium solutions responses elicited by repeated prolonged dopamine applications progressively declined. 4. It is suggested that the hyperpolarizing responses to dopamine depend on an influx of calcium into the cytosol from a store which can be replenished only from the exterior.

Sympathetically evoked secretory potentials in the parotid gland of the cat

1. In cats under chloralose anaesthesia micro-electrodes were inserted into parotid gland cells. 2. The average resting potential was found to be -35 . 6 +/- 4 . 7 (S.D.) mV. 3. Stimulation of the auriculo-temporal nerve caused hyperpolarizing, or occasionally depolarizing, secretory potentials of 5--10 mV, which were abolishable with atropine. 4. Stimulation of the cervical sympathetic trunk regularly caused, after long latency (several seconds), slow depolarizations of 15--20 mV, accompanied by a decrease in input resistance. They were antagonized by practolol and therefore assumed to be mediated by beta 1-adrenoceptors. Occasionally hyperpolarization, ascribed to an effect on alpha-adrenoceptors, was observed. 5. In many units the slow depolarization on sympathetic stimulation was preceded by short-lasting hyperpolarizing (sometimes depolarizing) transients. They resembled the evoked cholinergic responses but could be abolished not only by atropine but also by guanethidine.

Secretory response of dispersed rat submandibular cells. I. Potassium release

The secretory response of dispersed rat submandibular cells as it relates to the net efflux of K+ following sympathomimetic and parasympathomimetic stimulation was evaluated. The cholinergic agonists acetylcholine and carbamylcholine were found to have equal efficacy but dissimilar receptor affinity. Median effective concentrations (EC50's) were 1.7 x 10(-7) M and 7.1 x 10(-7) M, respectively. Pilocarpine was found to be acting as a "partial" agonist and had an EC50 at 5.6 x 10(-7) M. (-)-Norepinephrine and (-)-epinephrine were found to have similar efficacy and potency with EC50's at 5.6 x 10(-7) M and 3.8 x 10(-7) M, respectively. (-)-Phenylephrine was found to act as a partial agonist with an EC50 at 3.8 x 10(-6) M, whereas (-)-isoproterenol stimulation resulted in no net K+ efflux within the concentrations tested (10(-8) M--10(-3) M). Extracellular Ca2+, but not Mg2+, was shown to be required to elicit a net K efflux following either cholinergic or alpha-adrenergic stimulation.

A novel effect of cobalt treatment on calcium-dependent responses of the cockroach salivary gland

After incubation in calcium-free solutions containing cobalt, the readmission of calcium caused prolonged but reversible hyperpolarization of acinar cells of cockroach salivary glands and prolonged fluid secretion. It is suggested that cobalt treatment increases the permeability of the acinar cell membrane to calcium.

Mechanisms involved in alpha-adrenergic phenomena: role of calcium ions in actions of catecholamines in liver and other tissues

Epinephrine and norepinephrine binding sites with the physiological characteristics of alpha-adrenergic receptors have been identified in the plasma membranes of liver and other cells. Interaction of catecholamines with these receptors causes a mobilization of calcium ions from mitochondria and perhaps other intracellular stores in liver cells. In other cells, there may also be influx of extracellular calcium ions. Evidence is presented in support of the hypothesis that the rise in cytosolic calcium ions resulting from these changes is responsible for many of the alpha-adrenergic actions of catecholamines. Possible mechanisms by which activation of alpha-adrenergic receptors causes changes in calcium and other aspects of cellular metabolism are discussed.

Dynamics of beta adrenoceptor induced amylase release and cyclic AMP accumulation in the guinea pig submandibular gland

The dynamics of amylase release from the guinea pig submandibular gland were studied in vitro by applying a multi-channel microperfusion set. This technique makes it possible to measure time related enzyme release more accurately and to take samples of perifused tissue at short intervals. Stimulation of the beta-adrenoceptor with norepinephrine gives rise to a rapid initial enzyme discharge, detectable within 15 s. Administration of propranolol inhibits enzyme release, which is not restored after removal of the agent. Simultaneous measurements of tissue cyclic AMP during norepinephrine stimulation at various time intervals display a significant increase of cAMP as early as 15 s after stimulation of secretion. This increase of cAMP thus coincides with the discharge of amylase. In addition, cAMP continuously accumulates during 30 min of norepinephrine perifusion of the slices. The present study describes a valuable tool with high sensitivity for visualizing the relations between enzyme secretion from the salivary gland and the intracellular biochemical processes. The data obtained further indicate a close correlation between amylase and cAMP during the initial phase of enzyme discharge.

Alpha- and beta-adrenergic receptors of the rat salivary gland. Elevation after chemical sympathectomy

Binding of [3H]dihydroergokryptine and [3H]dihydroalprenolol to membrane preparations from rat submaxillary gland was measured to characterize the alpha- and beta-adrenergic receptors, respectively. Kinetic analysis of the data revealed a high affinity binding site for each radioligand. Inhibition of binding at each site was stereospecific for the active isomer of the catecholamine used. The greater ability of a beta1 than beta2 specific beta-adrenergic antagonist to displace [3H]dihydroalprenolol binding indicated that this binding site was of the beta1 type. Chemical sympathectomy with reserpine or 6-hydroxydopamine resulted in a significant increase in both [3H]dihydroalprenolol and [3H]dihydroergokryptine binding in the rat submaxillary gland. 3scatchard analysis of the data indicated that these increases in binding were due to a change in total number of binding sites for [3H]dihydroergokryptine and [3H]dihydroalprenolol with little change in apparent affinities. This suggests that changes in alpha- and beta-adrenergic receptor density may be important in the development of supersensitivity in salivary glands after reserpine and 6-hydroxydopamine treatment.

Parotid acinar cells: ionic dependence of acetylcholine-evoked membrane potential changes

Segments of mouse parotid were placed in a superfusion chamber. Surface acini were impaled by one or two micro-electrodes for measurement of membrane potential and resistance. The acinus under investigation was stimulated by micro-iontophoretic application of acetylcholine (ACh) or adrenaline. Neighbouring acinar cells were electrically coupled. Electrical coupling between acinar cells only occurred within restricted domains probably corresponding to an acinus or a group of acini. Passing direct current through one intracellular electrode, the resting potential of an acinus could be set at desired levels and the dependency of the ACh-evoked potential change on the resting potential investigated. The ACh null potential (initial effect) was about--60 mV. A delayed hyperpolarizing effect of ACh could not be reversed. The initial ACh-evoked potential change was sensitive to alterations in extracellular Na, K and Cl concentration. The delayed ACh-evoked hyperpolarization was blocked by ouabain, exposure to Na-free or K-free solutions. It is concluded that ACh increases mainly K and Na membrane conductance causing K efflux and Na influx with a subsequent Na activation of an electrogenic Na pump.

Membrane potential and resistance changes induced in salivary gland acinar cells by microiontophoretic application of acetylcholine and adrenergic agonists

The effects of microiontophoretic applications of catecholamines and acetylcholine on parotid acinar cell membrane potential and resistance were investigated using intracellular microelectrode recording in superfused segments of mouse parotid or rat submandibular glands. Short pulses of acetylcholine and alpha-adrenergic agonists had similar effects, consisting of a marked decrease in membrane resistance accompanied by an initial depolization or hyperpolarization depending on the level of the resting membrane potential. This initial response was followed by a slow hyperpolarization occurring at a time when the resistance was increasing towards the prestimulation level. The equilibrium potential for the initial potential change caused by excitation of the cholinergic receptors was investigated directly by setting the membrane potential at different levels by injecting direct current and stimulating the same cell repeatedly with equal doses of acetylcholine. The equilibrium potential was found to be about -55 mV. The delayed hyperpolarization could not be reversed by passing hyperpolarizing current, but actually increased in size with higher membrane potentials. The minimum latency of the effect of acetylcholine or alpha-adrenergic agonists was 200-500 msec. Excitation of beta-adrenoceptors caused, after a long latency of several seconds, a small depolarization. Epinephrine induced a combined alpha- and beta-adrenergic response, with the alpha-component predominating. Blocking the alpha-adrenoceptors with phentolamine revealed the beta-adrenergic depolarization, while blocking the beta-adrenoceptors with propranolol caused the components of the alpha-adrenergic response to become more pronounced. All three receptors (alpha- and beta-adrenoceptors and cholinergic receptors) were present in individual acini.

Electrical coupling and dye transfer between acinar cells in rat salivary glands

1. Adjacent acinar cells in isolated rat parotid and submaxillary glands were found to be electrically coupled in greater than 90% of the pairs tested. 2. Cells injected with fluorescein or procion yellow showed transfer of the dyes to their coupled neighbours. While not all coupled cells exchanged dye, exchange occurred only between coupled cells. 3. In experiments using three micro-electrodes, coupled acinar cells from parotid gland were found to have a mean coupling coefficient of 0.69 +/- 0.04. This value is higher than those reported for most other vertebrate epithelial systems. 4. Membrane damage sufficient to reduce the occurrence of coupling between cells by 97% lowered the transmembrane potential by only 13%. This would indicate that in this system membrane potential may not be the most sensitive indicator of cell damage. 5. The significance of the presence of electrical coupling and cell-to-cell transfer of small tracer molecules is discussed in relation to salivary gland structure and possible functional consequences.

Calcium ion uptake induced by cholinergic and alpha-adrenergic stimulation in isolated cells of rat salivary glands

Adrenaline (10(-5) M) and carbamylcholine (10(-4) M) stimulate 45Ca2+ uptake into isolated cells of rat submandibular galnd and parotid glands. In the presence of the alpha-adrenoreceptor blocking agent phentolamine, adrenaline stimulation of 45Ca2+ uptake is abolished. The beta-adrenergic stimulant isoproterenol has no effect on 45Ca2+ uptake. Carbamylcholine induced 45Ca2+ uptake is inhibited by atropine. The Ca2+ ionophore A23187 stimulates 45Ca2+ uptake, whereas dibutyryl cyclic adenosine 3',5'-monophosphate and dibutyryl cyclic guanosine 3',5'-monophosphate have no effect on 45Ca2+ uptake. A graphical analysis of the 45Ca2+ uptake curves reveals at least two phases: a fast phase and a slow phase, both of which are stimulated by adrenaline and carbamylcholine. The 45Ca-exchangeable pool size is increased by adrenaline and carbamylcholine in both the fast and the slow phases. These results suggest that alpha-adrenergic and cholinergic agonists act by increasing the rate of Ca2+ transfer into the cells of the parotid and submandibular salivary glands most probably through an increase of the cell membrane permeability for Ca2+.

The relationship between stimulation-induced potassium release and amylase secretion in the mouse parotid

1. The output of amylase from superfused mouse parotid segments in response to stimulation with acetylcholine (ACh), phenylephrine and isoprenaline during exposure to solutions with varying potassium concentrations was monitored by an on line automated fluorometric method. 2. During stimulation with ACh or phenylephrine a 10-fold increase in superfusion fluid potassium concentration caused an immediate very marked reduction in amylase output which was fully reversible. A 10-fold reduction in potassium concentration resulted in a prominent rise in amylase output. During stimulation with isoprenaline there was no effect on the amylase output of varying the extracellular potassium concentration. Acetylcholine and phenylephrine caused potassium release from the mouse parotid whereas isoprenaline had no such effect. 3. It appears that under conditions where stimulation-induced potassium release is enhanced there is also an enhanced amylase secretion and vice cersa. There may therefore be a link between passive potassium transport and amylase secretion.

Increase in membrane conductance by adrenaline in parotid acinar cells

It is shown that excitation of the alpha- or beta-adrenoceptors in mouse parotid acinar cells causes a marked reduction of surface cell membrane resistance. The alpha-adrenoceptor induced membrane effect is an increase in K conductance. The beta-adrenoceptor induced membrane effect does not seem to be mediated by cyclic AMP.

Membrane potential and resistance measurement in acinar cells from salivary glands in vitro: effect of acetylcholine

1. Cell membrane potential and input resistance measurements were made on segments of submaxillary glands from mice, rabbits or cats placed in a tissue bath, which was perfused with physiological salt solutions.2. During exposure to a standard Krebs-Henseleit solution, ACh stimulation always evoked a marked decrease in input resistance and time constant. The change in potential evoked by ACh stimulation was either a monophasic hyperpolarization (low resting potential) or a depolarization followed by hyperpolarization (high resting potential).3. Increasing [Ca](o) from 2.56 to 10 mM resulted in an enhanced input resistance. Under this condition it was sometimes possible to obtain current-voltage relations. The relationship was linear in the range -50 to -10 mV. In the absence of extracellular Ca the resting potential was reduced and ACh mostly evoked hyperpolarizations. In those cases when the resting potential remained high biphasic potentials were still observed.4. During exposure to Na-free solutions the resting potential was either unchanged or slightly enhanced. ACh never evoked biphasic potentials, but always large hyperpolarizations.5. In the first period (1 hr) after exposure to a K-free solution ACh normally evoked very large hyperpolarizations, often to more than -100 mV. After several hours of exposure to K-free solution the input resistance gradually increased and ACh evoked a tremendous fall in input resistance and time constant with only a small potential change. Re-introducing control solution, ([K](o) = 4.7) for a short period at this stage, caused a very marked hyperpolarization (about 30 mV) unaccompanied by a change in input resistance and time constant.6. Replacing extracellular Cl by SO(4) hyperpolarized the cell membrane. ACh mostly evoked hyperpolarization under this condition, but occasionally biphasic potentials were observed. Increasing [K] of the sulphate solution depolarized the cell membrane by about 49 mV per tenfold increase in [K]. In the presence of ACh the membrane behaved as a K-selective membrane with a slope of the linear curve relating membrane potential to [K](o) of 59 mV per tenfold increase in [K](o).7. It is concluded that ACh evokes a marked increase in surface cell membrane permeability of salivary acinar cells. The ACh evoked hyperpolarization is due to an increase in P(K): the depolarization frequently preceding the hyperpolarization is probably mainly related to an increase in P(Na). The membrane Na-K pump can act electrogenically at least under conditions of Na loading.