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

Regulation of membrane potential and fluid secretion by Ca2+-activated K+ channels in mouse submandibular glands

We have recently shown that the IK1 and maxi-K channels in parotid salivary gland acinar cells are encoded by the K(Ca)3.1 and K(Ca)1.1 genes, respectively, and in vivo stimulated parotid secretion is severely reduced in double-null mice. The current study tested whether submandibular acinar cell function also relies on these channels. We found that the K(+) currents in submandibular acinar cells have the biophysical and pharmacological footprints of IK1 and maxi-K channels and their molecular identities were confirmed by the loss of these currents in K(Ca)3.1- and K(Ca)1.1-null mice. Unexpectedly, the pilocarpine-stimulated in vivo fluid secretion from submandibular glands was essentially normal in double-null mice. This result and the possibility of side-effects of pilocarpine on the nervous system, led us to develop an ex vivo fluid secretion assay. Fluid secretion from the ex vivo assay was substantially (about 75%) reduced in animals with both K(+) channel genes ablated - strongly suggesting systemic complications with the in vivo assay. Additional experiments focusing on the membrane potential in isolated submandibular acinar cells revealed mechanistic details underlying fluid secretion in K(+) channel-deficient mice. The membrane potential of submandibular acinar cells from wild-type mice remained strongly hyperpolarized (-55 +/- 2 mV) relative to the Cl(-) equilibrium potential (-24 mV) during muscarinic stimulation. Similar hyperpolarizations were observed in K(Ca)3.1- and K(Ca)1.1-null mice (-51 +/- 3 and -48 +/- 3 mV, respectively), consistent with the normal fluid secretion produced ex vivo. In contrast, acinar cells from double K(Ca)3.1/K(Ca)1.1-null mice were only slightly hyperpolarized (-35 +/- 2 mV) also consistent with the ex vivo (but not in vivo) results. Finally, we found that the modest hyperpolarization of cells from the double-null mice was maintained by the electrogenic Na(+),K(+)-ATPase.

Muscarinic activation of Na+-dependent ion transporters and modulation by bicarbonate in rat submandibular gland acinus

To investigate the interaction between the ion channels and transporters in the salivary fluid secretion, we measured the membrane voltage (V(m)) and intracellular concentrations of Ca(2+), Na(+) ([Na(+)](c)), Cl(-), and H(+) (pH(i)) in rat submandibular gland acini (RSMGA). After a transient depolarization induced by a short application of acetylcholine (ACh; 5 muM, 20 s), RSMGA showed strong delayed hyperpolarization (V(h,ACh); -95 +/- 1.8 mV) that was abolished by ouabain. In the HCO(3)(-)-free condition, the V(h,ACh) was also blocked by bumetanide, a blocker of Na(+)-K(+)-2Cl(-) cotransporter (NKCC). In the presence of HCO(3)(-) (24 meq, bubbled with 5% CO(2)), however, the V(h,ACh) was not blocked by bumetanide, but it was suppressed by ethylisopropylamiloride (EIPA), a Na(+)/H(+) exchanger (NHE) inhibitor. Similarly, the ACh-induced increase in [Na(+)](c) was totally blocked by bumetanide in the absence of HCO(3)(-), but only by one-half in the presence of HCO(3)(-). ACh induced a prominent acidification of pH(i) in the presence of HCO(3)(-), and the acidification was further increased by EIPA treatment. Without HCO(3)(-), an application of ACh strongly accelerated the NKCC activity that was measured from the decay of pH(i) during the application of NH(4)(+) (20 mM). Notably, the ACh-induced activation of NKCC was largely suppressed in the presence of HCO(3)(-). In summary, the ACh-induced anion secretion in RSMGA is followed by the activation of NKCC and NHE, resulting an increase in [Na(+)](c). The intracellular Na(+)-induced activation of electrogenic Na(+)/K(+)-ATPase causes V(h,ACh). The regulation of NKCC and NHE by ACh is strongly affected by the physiological level of HCO(3)(-).

Ionic and pharmacologic characteristics of epithelial cells in a semi-intact preparation of the rat ventral prostate gland

BACKGROUND

The essential ionic and pharmacologic characteristics of epithelial cells within the ducts of the prostate gland are not well known.

METHODS

Experiments were carried out on segments of ventral prostate glands from adult male rats. By using sharp microelectrodes, intracellular epithelial cell and transepithelial (lumen) potentials were recorded in response to ionic substitution and application of ion channel blockers, hormones, and other pharmacologic agents related to prostatic function.

RESULTS

Membrane permeabilities to K(+), Na(+), and Cl(-) were found to account for approximately 43% of the resting membrane potential, whereas some 39% was likely to be metabolic in origin. The membrane potential also responded to adrenaline, acetylcholine, insulin, prolactin, testosterone, nerve growth factor, and nitric oxide. The lumen potential was found to be particularly sensitive to citrate, prolactin, and testosterone.

CONCLUSION

It was concluded that the basal membrane potential of prostatic epithelial cells is associated with a relatively high Na(+):K(+) permeability ratio and metabolic dependence. The hormonal and pharmacologic sensitivity observed is consistent with the functional characteristics of the prostate gland.

Visualization of the secretory process involved in Ca2+-activated fluid secretion from rat submandibular glands using the fluorescent dye, calcein

The central feature of fluid and electrolyte secretion by salivary acinar cells is transepithelial Cl- movement as a driving force for the secretion. However, little is known about the membrane localization and regulation by agonists of various anion channels. To characterize the anion transport and fluid secretion, we visualized the secretory process induced by the cholinergic agonist, carbachol (CCh), using the anionic fluorescent dye, calcein, under a confocal laser scanning microscope. The fluorescence of calcein loaded into the isolated acini was spread diffusely throughout the cytoplasm and was less intense in the secretory vesicles which occupied the apical pole. Cytoplasmic calcein was released into intercellular canaliculi just after the addition of CCh, depending upon a rise in [Ca2+]i by Ca2+ release from intracellular stores. Thereafter, the formation of watery vacuoles connected with intercellular canaliculi was visualized in the calcein-loaded acini, depending upon external Ca2+. Both the calcein release and vacuole formation were inhibited by suppressing the Ca(2+)-activated K+ efflux. The calcein release was also affected by the external anion substitution, suggesting that calcein is released through an anion channel. In the isolated, perfused glands, CCh-induced fluid secretion was sustained in two phases, whereas the loaded calcein was initially and transiently released into the saliva. By revealing the [Ca2+]i dependence and sensitivities to channel blockers, our results suggest that the initial phase of CCh-induced fluid secretion was evoked in association with the release of the organic anion, calcein, and the late phase of fluid secretion, during which calcein is less permeable, was associated with the formation of watery vacuoles. Thus, the anion channels possessing the distinct property of anion permeation may be activated in the initial phase and late phase. These results indicate that the anionic fluorescent dye, calcein, is useful for visualizing the process of Ca(2+)-dependent fluid secretion, and for clarifying the relation between fluid secretion and anion transport.

Stimulus-secretion coupling: cytoplasmic calcium signals and the control of ion channels in exocrine acinar cells

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Electrophysiologically distinct cell types in human sweat gland secretory coil

The human sweat gland secretory coil consists of three histologically distinct cell types: myoepithelial (ME), light (or clear), and dark cells. The electrophysiological properties of all these cells are poorly defined. Employing electrophysiological techniques, we report distinct pharmacological responses of three different cell types from freshly isolated human sweat gland secretory coil. The superficial ME cells are characterized by 1) spontaneous depolarizing spikes (2 to 50 mV), 2) high cell membrane potentials [Vm = -68.6 +/- 3.9 (SE) mV; n = 21], 3) a K(+)-selective cell membrane (slope response = 54.2 +/- 6.7 mV per decade K+ concentration; n = 4), 4) depolarizing responses to cholinergic agonist mecholyl (delta Vm = 29.1 +/- 3.1 mV, n = 21), and 5) insensitivity to beta-adrenergic stimulation (n = 12). Two other types of cells, presumably secretory, were also observed. We arbitrarily labeled these cells as beta-adrenergic sensitive (beta-S) and beta-adrenergic insensitive (beta-I) cells based on their respective sensitivity to isoproterenol (IPR), a beta-adrenomimetic. Properties of the beta-S cells include 1) relatively higher basolateral membrane potentials (Vm = -57.3 +/- 3.1 mV; n = 13), 2) depolarizing responses to IPR (delta Vm = 16.8 +/- 2.6 mV; n = 9) inhibitable by the beta-adrenergic antagonist propranolol, and 3) hyperpolarizing responses to mecholyl (delta Vm = -21.8 +/- 2.0 mV; n = 13). The beta-I cells are characterized by 1) low basolateral membrane potentials (Vm = -23.6 +/- 2.1 mV; n = 16), 2) insensitivity to beta-adrenergic stimulation, and 3) hyperpolarizating responses to mecholyl (delta Vm = -16.1 +/- 2.1 mV; n = 16).

Spatial distribution of intracellular, free Ca2+ in isolated rat parotid acini

The spatial distribution of intracellular, free Ca2+ ([Ca2+]i) in rat parotid acini was measured by imaging fura-2 fluorescence from individual acinar cells by means of a digital imaging microscope. Upon cholinergic stimulation in a Krebs-Ringer bicarbonate buffer at (37 degrees C), [Ca2+]i increased synchronously at both the basolateral and luminal membranes as well as in all cells of the secretory endpiece, reaching peak [Ca2+]i levels 1 s after stimulation. Atropine addition caused a rapid down-regulation of [Ca2+]i, which, however, never reached prestimulatory levels. When acini were stimulated in a medium containing 5 nM Ca2+, the Ca2+ mobilization arising from internal pools caused an increase in [Ca2+]i predominantly near the basolateral area, where the endoplasmic reticulum is located, and standing Ca2+ gradients were observed for up to 10 s. A mathematical model is developed to simulate the time courses of the Ca2+ profiles through the cytoplasm using estimated values of the Ca2+ diffusion coefficients and the cytosolic Ca2+ buffering capacity. It is concluded that under physiological conditions, the Ca2+ release from the endoplasmic reticulum is responsible for the activation of the basolaterally located K+ channels. Furthermore, Ca2+ influx from the interstitium is responsible for much of the rise in [Ca2+]i near the luminal membranes, where the Cl- channels are supposed to be located.

Apical membrane Na+/H+ exchange in Necturus gallbladder epithelium. Its dependence on extracellular and intracellular pH and on external Na+ concentration

Intracellular microelectrode techniques and extracellular pH measurements were used to study the dependence of apical Na+/H+ exchange on mucosal and intracellular pH and on mucosal solution Na+ concentration ([Na+]o). When mucosal solution pH (pHo) was decreased in gallbladders bathed in Na(+)-containing solutions, aNai fell. The effect of pHo is consistent with titration of a single site with an apparent pK of 6.29. In Na(+)-depleted tissues, increasing [Na+]o from 0 to values ranging from 2.5 to 110 mM increased aNai; the relationship was well described by Michaelis-Menten kinetics. The apparent Km was 15 mM at pHo 7.5 and increased to 134 mM at pHo 6.5, without change in Vmax. In Na(+)-depleted gallbladders, elevating [Na+]o from 0 to 25 mM increased aNai and pHi and caused acidification of a poorly buffered mucosal solution upon stopping the superfusion; lowering pHo inhibited both apical Na+ entry and mucosal solution acidification. Both effects can be ascribed to titration of a single site; the apparent pK's were 7.2 and 7.4, respectively. Diethylpyrocarbonate (DEPC), a histidine-specific reagent, reduced mucosal acidification by 58 +/- 4 or 39 +/- 6% when exposure to the drug was at pHo 7.5 or 6.5, respectively. Amiloride (1 mM) did not protect against the DEPC inhibition, but reduced both apical Na+ entry and mucosal acidification by 63 +/- 5 and 65 +/- 9%, respectively. In the Na(+)-depleted tissues mean pHi was 6.7. Cells were alkalinized by exposure to mucosal solutions containing high concentrations of nicotine or methylamine. Estimates of apical Na+ entry at varying pHi, upon increasing [Na+]o from 0 to 25 mM, indicate that Na+/H+ exchange is active at pHi 7.4. Intracellular H+ stimulated apical Na+ entry by titration of more than one site (apparent pK 7.1, Hill coefficient 1.7). The results suggest that external Na+ and H+ interact with one site of the Na+/H+ exchanger and that cytoplasmic H+ acts on at least two sites. The external titratable group seems to be an imidazolium, which is apparently different from the amiloride-binding site. The dependence of Na+ entry on pHi supports the notion that the Na+/H+ exchanger is operational under normal transport conditions.

Cholinergic-induced electrolyte transport in rat parotid acini

Secretory responses of parotid acini occurring within 10 sec following cholinergic stimulation were characterized. 1. Measurement of membrane potentials by means of the fluorescent dye diSC3-(5) revealed a value of approximately -59 mV, which remained unaffected on stimulation. 2. Stimulation caused a rapid net loss of 42K+ that was strongly inhibited by the "maxi" K+-channel inhibitor "charybdotoxin" present in scorpion venom. 3. It was calculated that the number of open "maxi" K+-channels per cell was approximately 40 in the unstimulated state and approximately 3000 in the stimulated state. 4. Stimulation caused a transient decrease in the acinar ATP content. 5. Intracellular pH (pHi) measured by means of the fluorescent dye, BCECF, was dependent upon the presence of extracellular HCO3- as well as Na+. Under physiological conditions pHi was 7.27 and stimulation caused a transient decrease of 0.1 pH units due to HCO3- efflux. The decrease was followed by pHi recovery mediated by a Na+/H+ exchange mechanism.

MSORTV imaging of electrotonic conduction in a syncitium: optical recording of polarization spread in a simple salivary gland

The spread of hyperpolarizing current injected into the salivary gland of the snail Helisoma trivolvis was studied with the technique of multiple site optical recording of transmembrane voltage (MSORTV). Using MSORTV it was possible to monitor simultaneously the spread of current passed by an intracellular microelectrode in up to 124 separate gland regions. Our results demonstrate the feasibility of combining conventional intracellular injection techniques with MSORTV to visualize 2-dimensional current spread in an electrical syncitium.

A patch-clamp study of potassium currents in resting and acetylcholine-stimulated mouse submandibular acinar cells

Salivary acini were enzymatically isolated from submandibular glands of adult male mice. The patch-clamp technique was employed to investigate the conductive properties and activities of a large-conductance K+ channel in both cell-attached and in excised patches of basolateral acinar cell membranes. In excised, inside out, patches with identical high-K+ solutions (145 mM-KCl) on either side of the membrane the current-voltage (I-V) plot was linear. The mean single-channel conductance was 245 +/- 4.8 ps with a single-channel permeability of 4.6 X 10(-13) cm3 s-1. At Ca2+ concentrations of 10(-9)-10(-8) M bathing the intracellular membrane face the channel was exquisitely sensitive to changes in transmembrane potential, i.e. voltage sensitive. At 10(-7) M-Ca the channel was almost always open and displayed little sensitivity to voltage. Single-channel currents were recorded in cell-attached patches. When the recording pipettes contained the high-K+ solution the I-V plots were linear and the mean single-channel conductance and permeability almost identical to that in the excised patches. The mean spontaneous resting potential of the acinar cells bathed in physiological saline (140 mM-NaCl, 4.5 mM-KCl) was -43 +/- 1.8 mV. The voltage sensitivity of the in situ K+ channel was very similar to that recorded in excised patches at 10(-9)-10(-8) M-Ca. In experiments designed to reproduce the physiological ionic gradients across the patch membrane pipettes were filled with the high-Na+ solution. The I-V plot was not linear but showed pronounced rectification at negative membrane potentials. The channel is K+ selective and the extrapolated reversal potential was close to -90 mV. The single-channel conductance at the spontaneous resting membrane potential was about 35 pS. The single-channel permeability was however only slightly reduced at 4.29 X 10(-13) cm3 s-1. It was demonstrated that current flow through the open K+ channel could be accurately modelled using constant field electrodiffusion theory. Continuous in situ recordings before and after application of the agonist acetylcholine to the solution bathing the acini revealed that acetylcholine stimulation is associated with a marked increase in the frequency and duration of K+ currents in the patch membrane. The increased current activity in the patch membrane during acetylcholine application must be mediated via an intracellular second messenger and was very similar to that observed in the excised patches on increasing ionized Ca2+ concentrations from 10(-8) to 10(-7) M.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional gap junctions in mouse small intestinal crypts

We demonstrate intercellular transfer of Lucifer yellow and the existence of gap junctions in isolated mouse small intestinal crypts. The pattern of dye transfer approximates the normal pattern of cell proliferation and differentiation in the intestinal epithelium. These findings indicate that the cells of normal crypts form an effective intercellular continuum. This intercellular route may function in the establishment of chemical, ionic, or electrical fields, which in turn may play a role in the control of cell proliferation, differentiation, and secretion in the crypt.

Three types of calcium-dependent channel in rat lacrimal glands

Isolated cells from rat lacrimal glands were studied with patch-clamp techniques. Whole-cell and cell-attached recordings were obtained while the cells were stimulated by application of carbamylcholine or of the Ca ionophore, A23187. The results were compared with recordings of Ca-dependent channels obtained in isolated patches. Whole-cell recordings revealed two types of carbamylcholine-induced current. At low levels of stimulation, a specific class of Ca-dependent K channels was selectively activated ('BK channels'). With more intense stimulation an inward current, Ii, was obtained at the cell resting potential. Ii rose rather abruptly after a long delay. In several cells, Ii currents presented spontaneous oscillations. Both K and Ii current responses to carbamylcholine were due to activation of muscarinic receptors. Both responses were elicited by a rise of the intracellular Ca concentration. The immediate source of Ca was intracellular. Replacement of intracellular K with either Na or Cs blocked BK channels entirely, thus allowing the study of Ii currents free from K currents. Ii responses to carbamylcholine were, however, less frequently obtained in Na- or Cs-dialysed cells than in K-dialysed cells. In symmetrical NaCl solutions, Ii inverted at 0 mV. When replacing part of the intracellular or extracellular Cl with glutamate the reversal potential, Ei, was found to vary in the same direction as the equilibrium potential for Cl ions, ECl. In some experiments, Ei was close to ECl but in others Ei deviated strongly from ECl. These experiments suggested that Ii was mainly due to a Cl-selective conductance, and that another conductance type was contributing to Ii in variable proportions. It was found that, in K-free solutions, Ii had a reversal potential very close to ECl. Noise analysis showed that the Cl channels involved in Ii current had a unit conductance of about 1-2 pS in symmetrical NaCl solutions. At -60 mV, the mean channel open time derived from noise power spectra was about 200 ms. The activation of the Ca-dependent Cl channels was increased by depolarization. Voltage jumps elicited slow exponential relaxations. At -60 mV, the time constants of the relaxations were in the range 100-250 ms. Cell-attached recordings suggested that internal Ca activated three types of channel, depending on the Ca concentration: BK channels, 2-4 pS channels and 25 pS channels. Inside-out and outside-out patch conditions allowed a rough estimate to be made of the Ca concentration needed to activate each class of channel.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical activity of an intestinal epithelial cell line: hyperpolarizing responses to intestinal secretagogues

Cultured epithelial cells (Intestine 407) derived from fetal human small intestine exhibited spontaneous oscillations of membrane potential between the resting level of about -20 mV and the activated level of about -75 mV. The cells were hyperpolarized to the latter level in response to mechanical or electrical stimuli. The hyperpolarizing responses were also elicited by the application of intestinal secretagogues: acetylcholine, histamine, serotonin and vasoactive intestinal polypeptide (VIP). The spontaneous oscillation of membrane potential became prominent and long-lasting in the presence of acetylcholine, histamine, serotonin or VIP. These secretagogue-induced responses were mediated by individual independent receptors on the cell membrane. Muscarinic receptors were responsible for the acetylcholine response, and H1-receptors for the histamine response. The cells also responded with a slow hyperpolarization to calcium ionophore A23187, which is known to induce intestinal secretion. The spontaneously occurring hyperpolarizing responses and those induced by stimuli were both due to an increase in the K+ conductance of the cell membrane. Since acetylcholine, histamine, serotonin and A23187 are known to promote mobilization of cellular Ca2+ ions in intestinal secretory cells, it is hypothesized that these electrical activities of the cell are closely related to the receptor stimulation which leads to the Ca2+-mediated intestinal secretion.

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)

Effect of extracellular K+ on saliva secretion by isolated, perfused rat submandibular glands

Changes in perfusate K+ concentration altered the secretory response of the glands to 10(-6) M acetylcholine. Lack of extracellular K+ caused a transient fluid secretory response lasting less than 10 min, and a 91 per cent reduction in the overall volume of saliva secreted in 60 min; it inhibited the response to acetylcholine even when the perfusate was changed to K+-containing solutions after 30 min. Absence of K+ in the perfusate resulted in increased Na+ and decreased K+ and Cl- concentrations in saliva. An increase in the perfusate K+ concentration to 50 mM/l caused a reduced but more sustained secretory response, although the volume of saliva secreted in 60 min was still reduced by 76 per cent compared to that obtained when the perfusate contained 4.6 m-equiv./l K+. Acetylcholine release induced by the high K seemed mostly responsible as the response was inhibited by atropine. However, in the presence of excess of exogenous acetylcholine, perfusion with high K+ medium resulted in reduced Na+ and elevated K+ and Cl- concentrations in saliva. It seems that a physiological (4-5 m-equiv./l) extracellular K+ concentration is required for acinar fluid secretion and for transductal electrolyte transport in the rat glands; lack of external K+ hyperpolarizes salivary cells and, although allowing an initial increase in Na+ conductance capable of causing secretion, prevents the further influx of this ion required to sustain saliva secretion. It also inhibits K+ secretion and Na+ re-absorption in salivary ducts, probably by inhibiting the Na+, K+ pump in duct cells; high external K+ depolarizes acinar cells and may reduce Na+ conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Multiple-site optical recording of membrane potential from a salivary gland. Interaction of synaptic and electrotonic excitation

The interaction between synaptic and electronic excitation of cells from the salivary gland of the snail Helisoma trivolvis was studied using a voltage-sensitive merocyanine dye. Linear and square photodiode matrix arrays were used to record simultaneously the response to neuronal stimulation of 15-25 separate regions of the gland. Laterally opposed acini exhibited highly synchronous electrical activity, which suggested a correspondingly high degree of electrical coupling. In the longitudinal direction, coupling appeared weaker. The onset of depolarization after neuronal stimulation was progressively delayed along the longitudinal gland axis, in agreement with the measured conduction velocity of the presynaptic nerve spike. In most instances, neuronal stimulation directly activated a regenerative gland response (action potential) at the junction between the anterior and central duct. Excitation of distal gland regions was usually mediated by electronic spread from active, more proximal gland regions. Occasionally, "collisions" between excitatory waves traveling in opposite directions were observed.

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.

Direct intercellular communication in health and disease: an overview

Direct intercellular communication (cell to cell coupling) is a mechanism for the local transit of information between cells and supplements the endocrine and nervous systems. Electrophysiological, biochemical, histological and cell culture techniques have established the widespread existence of coupling in mammalian tissues, and the importance of the gap junction has been recognised. Information is carried in the form of ions and small molecules between cells, and sensitive apparatus exists within each cell for controlling the permeability of the junctional membrane. The system may be important in the control and co-ordination of cellular metabolism and growth in the embryo and in adult tissues. Disorders of direct intercellular communication may be important in the pathogenesis of some diseases, in particular cancer.

Dissociation between stimulant-evoked acinar membrane resistance change and amylase secretion in the mouse parotid gland

1. Isolated segments of mouse parotid gland were superfused with a physiological saline solution. Membrane potential and input resistance were measured with intracellular micro-electrodes. Amylase secretion was monitored using an automated fluorometric assay. The parotid gland was stimulated by exposure to isoprenaline (10(-6)M), ACh (10(-6) M), dibutyryl or monobutyryl cyclic AMP (10(-3)M). 2. ACh evoked a sharp decrease in input resistance (from about 3-6 M omega to 1-2 M omega ) accompanied by a modest (two-fold) increase in amylase output. Both effects were fully reversible. 3. Isoprenaline evoked a marked increase in amylase output (five to ten-fold) reaching maximum after 20-30 min of stimulation. Throughout the period of intensive secretion the input resistance remained at the control level. Short pulses of ACh stimulation dramatically reduced input resistance in this period. 4. Dibutyryl cyclic AMP and monobutyryl cyclic AMP caused marked increases in amylase output (five to ten-fold) peaking 40-50 min after start of the continuous stimulation. This increase in amylase secretion was not accompanied by any change in input resistance. Short pulses of ACh stimulation sharply and reversibly reduced input resistance both in control and test periods. 5. It is concluded that the ACh-evoked reduction in input resistance is not a consequence of the secretory event and that secretion does not itself cause a decrease in input resistance.

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.

Substance P: indirect and direct effects on parotid acinar cell membrane potential

Direct effects of exogenously applied substance P on salivary acinar cells have been previously reported. This electro-physiological study confirms these direct effects in rat but not mouse parotid gland. We demonstrate that in the absence of autonomic blockade the peptide evokes marked responses which are blocked by atropine (10(-6) M). These effects cannot be attributed to direct activation of acinar cells and are presumably due to release of acetylcholine from parasympathetic nerve endings. We consider that substance P, in addition to direct effects, could act to modulate neuronal activity in salivary glands; a role already assigned to the peptide in the central nervous system.

Electrophysiology of mouse parotid acini: effects of electrical field stimulation and ionophoresis of neurotransmitters

1. Intracellular micro-electrode recordings of membrane potential and input resistance were made from surface acini of mouse parotid glands placed in a Perspex tissue bath through which oxygenated physiological saline solutions were circulated. The acinar cells were stimulated by microionophoresis of both acetylcholine (ACh) and adrenaline (Ad) from extracellular micropipettes, and by electrical field stimulation via a pair of platinum electrodes. 2. The acinar cells had a mean resting membrane potential of -64.9 mV +/- 0.6 S.E. The input resistance of the unstimulated cell was 4.63 M omega +/- 0.19 S.E. In a number of cells spontaneous miniature depolarizations were observed, associated with synchronous reductions in input resistance. 3. The responses to ionophoresis of both ACh and Ad and the response to supra-maximal field stimulation were identical. Stimulation always evoked a marked decrease in input resistance associated with an initial potential change, generally followed by a delayed hyperpolarization during which the input resistance returned to normal. 4. Field-stimulation responses could be evoked to single shock (1-2 msec) and to low frequency (1-4 Hz) stimulation. The latency for this response was 245 msec +/- 12 S.E. 5. The field-stimulation response was shown to be susceptible to blockade of nerve conduction in sodium-free or tetrodotoxin- (TTX-) containing media; and to blockade of neurotransmitter release in calcium-free media. 6. The field-stimulation and ACh responses were recorded at different levels of membrane potential within the same cells by applying either hyperpolarizing or depolarizing direct current through the recording electrode. The membrane potential at which the initial potential change undergoes reversal, i.e. changes from a depolarization to a hyperpolarization, is known as the equilibrium or reversal potential, EFS and EACh respectively. The field-stimulation (FS) and ACh responses underwent simultaneous reversal at about -60 mV, i.e. EFS = EACh. Equilibrium potentials were also determined indirectly by analysis of the responses evoked by each stimulus in the manner described by Trautwein & Dudel (1958). Using this technique the equilibrium potentials of the responses to all three stimuli, field stimulation, ACh and Ad, were again about -60mV, i.e. EFS = EACh = EAd. 7. Both the field-stimulation and ACh responses were abolished by atrophine (10(-6) M) while the response to Ad persisted. Atropine also abolished all spontaneous activity. The alpha-adrenergic blocker phentolamine (10(-5) M) abolished the response to Ad but left the field-stimulation response unaffected. 8. Electrical field stimulation of isolated segments of salivary gland evoked release of endogenous neurotransmitter as a consequence of neural excitation. The technique of field stimulation thus makes it possible to investigate the functional innervation of a gland using the in vitro preparation. In the mouse parotid gland the field stimulus response was mediated by ACh released from parasympathetic nerve endings.

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).

Substance P increases membrane conductance in parotid acinar cells

Substance P, a naturally occuring polypeptide of mammalian origin, has been implicated both as a neurotransmitter and a neurohormone. However, little is known of the ionic mechanisms underlying the postsynaptic response to substance P. In myenteric neurones Katayama and North reported substance P-evoked depolarisations (of varying latency) associated, surprisingly, with a decrease in membrane conductance. A direct measurement of reversal potential in normal ionic conditions was not achieved but it was suggested that substance P acts by reducing membrane potassium conductance. In contrast, work on salivary glands suggests that substance P evokes an increase in potassium conductance; however,electrophysiological work has not been carried out to verify this. We report here that substance P evokes a marked increase in rat parotid acinar cell membrane conductance associated with a potential change (latency 1.7 s) that reverses at about --65 mV. The reversal potential for substance P is shown to be identical to that obtained in the same cells for acetylcholine (ACh) and adrenaline. The identical membrane action of ACh, adrenaline and substance P, mediated by three separate receptor sites, suggests activation of a common effector mechanism.

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

The effect of cholinergic neural excitation by field stimulation on the acinar cell membrane potential was investigated in superfused segments of mouse pancreas and salivary glands (sublingual, submaxillary, and parotid glands). Responses of acinar cells in both exocrine pancreas and salivary glands to the neural excitation obtained by field stimulation were similar to responses previously described in each gland to the externally applied acetylcholine. In the pancreatic acinar cell, electrical field simulation induced depolarization with a latency of 0.3 to 1.2 sec. This depolarization was accompanied by a marked decrease in membrane resistance. The equilibrium potential of the depolarization induced by stimulation was between - 10 and - 20 mV. In the sublingual gland, field stimulation induced depolarization of the acinar cell with a latency of 0.2 to 0.3 sec. The stimulus induced depolarization was block by the addition of atropine. In the submaxillary and parotid glands, field stimulation induced depolarization in some acinar cell and hyperpolarization in other cells. The results support evidence previously presented by Petersen and his colleagues that acetylcholine acts to increase Na+ and K+ or Na+, K+, and Cl- permeabilities in the pancreatic acinar cell and to increase K+ and Na+ permeabilities in the salivary gland [11,24].

Direct visualization of cell to cell coupling: transfer of fluorescent probes in living mammalian pancreatic acini

A technique whereby it is possible directly to observe the movement of organic molecules from cell to cell in living mammalian exocrine glands is described. Thin translucent segments of mouse pancreas are mounted in a superfusion bath. Fluorescent probes are injected intracellularly via fine micropipettes and fluorescence observed. Both fluorescein (mol. wt. 332) and procion yellow (mol. wt. 697) are shown to be transferred from the injection cell to neighbouring acinar cells. This shows directly the existence of intercellular communicating pathways.