Bidirectional modulation of exocytosis by angiotensin II involves multiple G-protein-regulated transduction pathways in chromaffin cells

Membrane Capacitance Measurements of Stimulus-Evoked Exocytosis in Adrenal Chromaffin Cells

Research using membrane capacitance (C_m) measurements in adrenal chromaffin cells has transformed our understanding of the molecular mechanisms controlling regulated exocytosis. This is in part due to the exquisite temporal resolution of the technique, and the possibility of combining quantification of exo-/endocytosis at the whole-cell level, with the ability to simultaneously monitor and control the calcium signals triggering vesicle fusion. In this regard, experiments performed with C_m measurements complement amperometry experiments that give a measure of secreted transmitter and the behavior of the fusion pore, and fluorescent microscopy studies used to monitor vesicle and protein dynamics in imaged regions of the cell. In this chapter, we provide a detailed account of the methodology used to perform whole-cell patch clamp measurements of C_m in combination with voltage-clamp recordings of voltage-gated calcium channels to quantify stimulus-secretion coupling in chromaffin cells. Stimulus protocols developed for investigation of functionally distinct releasable vesicle pools are also described.

PTP-MEG2 regulates quantal size and fusion pore opening through two distinct structural bases and substrates

Tyrosine phosphorylation of secretion machinery proteins is a crucial regulatory mechanism for exocytosis. However, the participation of protein tyrosine phosphatases (PTPs) in different exocytosis stages has not been defined. Here we demonstrate that PTP-MEG2 controls multiple steps of catecholamine secretion. Biochemical and crystallographic analyses reveal key residues that govern the interaction between PTP-MEG2 and its substrate, a peptide containing the phosphorylated NSF-pY⁸³ site, specify PTP-MEG2 substrate selectivity, and modulate the fusion of catecholamine-containing vesicles. Unexpectedly, delineation of PTP-MEG2 mutants along with the NSF binding interface reveals that PTP-MEG2 controls the fusion pore opening through NSF independent mechanisms. Utilizing bioinformatics search and biochemical and electrochemical screening approaches, we uncover that PTP-MEG2 regulates the opening and extension of the fusion pore by dephosphorylating the DYNAMIN2-pY¹²⁵ and MUNC18-1-pY¹⁴⁵ sites. Further structural and biochemical analyses confirmed the interaction of PTP-MEG2 with MUNC18-1-pY¹⁴⁵ or DYNAMIN2-pY¹²⁵ through a distinct structural basis compared with that of the NSF-pY⁸³ site. Our studies thus provide mechanistic insights in complex exocytosis processes.

Ca2+ influx and clearance at hyperpolarized membrane potentials modulate spontaneous and stimulated exocytosis in neuroendocrine cells

Neuroendocrine adrenal chromaffin cells release neurohormones catecholamines in response to Ca²⁺ entry via voltage-gated Ca²⁺ channels (VGCCs). Adrenal chromaffin cells also express non-voltage-gated channels, which may conduct Ca²⁺ at negative membrane potentials, whose role in regulation of exocytosis is poorly understood. We explored how modulation of Ca²⁺ influx at negative membrane potentials affects basal cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and exocytosis in metabolically intact voltage-clamped bovine adrenal chromaffin cells. We found that in these cells, Ca²⁺ entry at negative membrane potentials is balanced by Ca²⁺ extrusion by the Na⁺/Ca²⁺ exchanger and that this balance can be altered by membrane hyperpolarization or stimulation with an inflammatory hormone bradykinin. Membrane hyperpolarization or application of bradykinin augmented Ca²⁺-carrying current at negative membrane potentials, elevated basal [Ca²⁺]_i, and facilitated synchronous exocytosis evoked by the small amounts of Ca²⁺ injected into the cell via VGCCs (up to 20 pC). Exocytotic responses evoked by the injections of the larger amounts of Ca²⁺ via VGCCs (> 20 pC) were suppressed by preceding hyperpolarization. In the absence of Ca²⁺ entry via VGCCs and Ca²⁺ extrusion via the Na⁺/Ca²⁺ exchanger, membrane hyperpolarization induced a significant elevation in [Ca²⁺]_i and asynchronous exocytosis. Our results indicate that physiological interferences, such as membrane hyperpolarization and/or activation of non-voltage-gated Ca²⁺ channels, modulate basal [Ca²⁺]_i and, consequently, segregation of exocytotic vesicles and their readiness to be released spontaneously and in response to Ca²⁺ entry via VGCCs. These mechanisms may play role in homeostatic plasticity of neuronal and endocrine cells.

Doc2B acts as a calcium sensor for vesicle priming requiring synaptotagmin-1, Munc13-2 and SNAREs

Doc2B is a cytosolic protein with binding sites for Munc13 and Tctex-1 (dynein light chain), and two C2-domains that bind to phospholipids, Ca²⁺ and SNAREs. Whether Doc2B functions as a calcium sensor akin to synaptotagmins, or in other calcium-independent or calcium-dependent capacities is debated. We here show by mutation and overexpression that Doc2B plays distinct roles in two sequential priming steps in mouse adrenal chromaffin cells. Mutating Ca²⁺-coordinating aspartates in the C2A-domain localizes Doc2B permanently at the plasma membrane, and renders an upstream priming step Ca²⁺-independent, whereas a separate function in downstream priming depends on SNARE-binding, Ca²⁺-binding to the C2B-domain of Doc2B, interaction with ubMunc13-2 and the presence of synaptotagmin-1. Another function of Doc2B – inhibition of release during sustained calcium elevations – depends on an overlapping protein domain (the MID-domain), but is separate from its Ca²⁺-dependent priming function. We conclude that Doc2B acts as a vesicle priming protein.

Molecular mechanism for muscarinic M1 receptor-mediated endocytosis of TWIK-related acid-sensitive K+ 1 channels in rat adrenal medullary cells

KEY POINTS

The muscarinic acetylcholine receptor (mAChR)-mediated increase in excitability in rat adrenal medullary cells is at least in part due to inhibition of TWIK (tandem of P domains in a weak inwardly rectifying K⁺ channel)-related acid-sensitive K⁺ (TASK)1 channels. In this study we focused on the molecular mechanism of mAChR-mediated inhibition of TASK1 channels. Exposure to muscarine resulted in a clathrin-dependent endocytosis of TASK1 channels following activation of the muscarinic M₁ receptor (M₁ R). This muscarinic signal for the endocytosis was mediated in sequence by phospholipase C (PLC), protein kinase C (PKC), and then the non-receptor tyrosine kinase Src with the consequent tyrosine phosphorylation of TASK1. The present results establish that TASK1 channels are tyrosine phosphorylated and internalized in a clathrin-dependent manner in response to M₁ R stimulation and this translocation is at least in part responsible for muscarinic inhibition of TASK1 channels in rat AM cells.

ABSTRACT

Activation of muscarinic receptor (mAChR) in rat adrenal medullary (AM) cells induces depolarization through the inhibition of TWIK-related acid-sensitive K⁺ (TASK)1 channels. Here, pharmacological and immunological approaches were used to elucidate the molecular mechanism for this mAChR-mediated inhibition. TASK1-like immunoreactive (IR) material was mainly located at the cell periphery in dissociated rat AM cells, and its majority was internalized in response to muscarine. The muscarine-induced inward current and translocation of TASK1 were suppressed by dynasore, a dynamin inhibitor. The muscarinic translocation was suppressed by MT7, a specific M₁ antagonist, and the dose-response curves for muscarinic agonist-induced translocation were similar to those for the muscarinic inhibition of TASK1 currents. The muscarine-induced inward current and/or translocation of TASK1 were suppressed by inhibitors for phospholipase C (PLC), protein kinase C (PKC), and/or Src. TASK1 channels in AM cells and PC12 cells were transiently associated with Src and were tyrosine phosphorylated in response to muscarinic stimulation. After internalization, TASK1 channels were quickly dephosphorylated even while they remained in the cytoplasm. The cytoplasmic TASK1-like IR material quickly recycled back to the cell periphery after muscarine stimulation for 0.5 min, but not 10 min. We conclude that M₁ R stimulation results in internalization of TASK1 channels through the PLC-PKC-Src pathway with the consequent phosphorylation of tyrosine and that this M₁ R-mediated internalization is at least in part responsible for muscarinic inhibition of TASK1 channels in rat AM cells.

Arrestin-biased AT1R agonism induces acute catecholamine secretion through TRPC3 coupling

Acute hormone secretion triggered by G protein-coupled receptor (GPCR) activation underlies many fundamental physiological processes. GPCR signalling is negatively regulated by β-arrestins, adaptor molecules that also activate different intracellular signalling pathways. Here we reveal that TRV120027, a β-arrestin-1-biased agonist of the angiotensin II receptor type 1 (AT1R), stimulates acute catecholamine secretion through coupling with the transient receptor potential cation channel subfamily C 3 (TRPC3). We show that TRV120027 promotes the recruitment of TRPC3 or phosphoinositide-specific phospholipase C (PLCγ) to the AT1R-β-arrestin-1 signalling complex. Replacing the C-terminal region of β-arrestin-1 with its counterpart on β-arrestin-2 or using a specific TAT-P1 peptide to block the interaction between β-arrestin-1 and PLCγ abolishes TRV120027-induced TRPC3 activation. Taken together, our results show that the GPCR-arrestin complex initiates non-desensitized signalling at the plasma membrane by coupling with ion channels. This fast communication pathway might be a common mechanism of several cellular processes.

Muscarinic inhibition of nicotinic transmission in rat sympathetic neurons and adrenal chromaffin cells

Little is known about the interactions between nicotinic and muscarinic acetylcholine receptors (nAChRs and mAChRs). Here we report that methacholine (MCh), a selective agonist of mAChRs, inhibited up to 80% of nicotine-induced nAChR currents in sympathetic superior cervical ganglion neurons and adrenal chromaffin cells. The muscarine-induced inhibition (MiI) substantially reduced ACh-induced membrane currents through nAChRs and quantal neurotransmitter release. The MiI was time- and temperature-dependent. The slow recovery of nAChR current after washout of MCh, as well as the high value of Q10 (3.2), suggested, instead of a direct open-channel blockade, an intracellular metabotropic process. The effects of GTP-γ-S, GDP-β-S and pertussis toxin suggested that MiI was mediated by G-protein signalling. Inhibitors of protein kinase C (bisindolymaleimide-Bis), protein kinase A (H89) and PIP2 depletion attenuated the MiI, indicating that a second messenger pathway is involved in this process. Taken together, these data suggest that mAChRs negatively modulated nAChRs via a G-protein-mediated second messenger pathway. The time dependence suggests that MiI may provide a novel mechanism for post-synaptic adaptation in all cells/neurons and synapses expressing both types of AChRs.

Lactate-mediated glia-neuronal signalling in the mammalian brain

Astrocytes produce and release L-lactate as a potential source of energy for neurons. Here we present evidence that L-lactate, independently of its caloric value, serves as an astrocytic signalling molecule in the locus coeruleus (LC). The LC is the principal source of norepinephrine to the frontal brain and thus one of the most influential modulatory centers of the brain. Optogenetically activated astrocytes release L-lactate, which excites LC neurons and triggers release of norepinephrine. Exogenous L-lactate within the physiologically relevant concentration range mimics these effects. L-lactate effects are concentration-dependent, stereo-selective, independent of L-lactate uptake into neurons and involve a cAMP-mediated step. In vivo injections of L-lactate in the LC evokes arousal similar to the excitatory transmitter, L-glutamate. Our results imply the existence of an unknown receptor for this ‘glio-transmitter’.

The astrocytic release of the metabolite L-lactate is implicated in modulating neuronal activity in the brain. Here, the authors show that L-lactate released from astrocytes excites noradrenergic neurons in the locus coeruleus and triggers the release of noradrenaline, increasing network excitability.

Identification of muscarinic receptor subtypes involved in catecholamine secretion in adrenal medullary chromaffin cells by genetic deletion

BACKGROUND AND PURPOSE

Activation of muscarinic receptors results in catecholamine secretion in adrenal chromaffin cells in many mammals, and muscarinic receptors partly mediate synaptic transmission from the splanchnic nerve, at least in guinea pigs. To elucidate the physiological functions of muscarinic receptors in chromaffin cells, it is necessary to identify the muscarinic receptor subtypes involved in excitation.

EXPERIMENTAL APPROACH

To identify muscarinic receptors, pharmacological tools and strains of mice where one or several muscarinic receptor subtypes were genetically deleted were used. Cellular responses to muscarinic stimulation in isolated chromaffin cells were studied with the patch clamp technique and amperometry.

KEY RESULTS

Muscarinic M₁, M₄ and M₅ receptors were immunologically detected in mouse chromaffin cells, and these receptors disappeared after the appropriate gene deletion. Mouse cells secreted catecholamines in response to muscarinic agonists, angiotensin II and a decrease in external pH. Genetic deletion of M₁, but not M₃, M₄ or M₅, receptors in mice abolished secretion in response to muscarine, but not to other stimuli. The muscarine-induced secretion was suppressed by MT7, a snake peptide toxin specific for M₁ receptors. Similarly, muscarine failed to induce an inward current in the presence of MT7 in mouse and rat chromaffin cells. The binding affinity of VU0255035 for the inhibition of muscarine-induced currents agreed with that for the M₁ receptor.

CONCLUSIONS AND IMPLICATIONS

Based upon the effects of genetic deletion of muscarinic receptors and MT7, it is concluded that the M₁ receptor alone is responsible for muscarine-induced catecholamine secretion.

β1-Blockers Lower Norepinephrine Release by Inhibiting Presynaptic, Facilitating β1-Adrenoceptors in Normotensive and Hypertensive Rats

Peripheral norepinephrine release is facilitated by presynaptic β-adrenoceptors, believed to involve the β₂-subtype exclusively. However, β₁-selective blockers are the most commonly used β-blockers in hypertension. Here the author tested the hypothesis that β₁AR may function as presynaptic, release-facilitating auto-receptors. Since β₁AR-blockers are injected during myocardial infarction, their influence on the cardiovascular response to acute norepinephrine release was also studied. By a newly established method, using tyramine-stimulated release through the norepinephrine transporter (NET), presynaptic control of catecholamine release was studied in normotensive and spontaneously hypertensive rats. β₁AR-selective antagonists (CGP20712A, atenolol, metoprolol) reduced norepinephrine overflow to plasma equally efficient as β₂AR-selective (ICI-118551) and β₁₊₂AR (nadolol) antagonists in both strains. Neither antagonist lowered epinephrine secretion. Atenolol, which does not cross the blood–brain barrier, reduced norepinephrine overflow after adrenalectomy (AdrX), AdrX + ganglion blockade, losartan, or nephrectomy. Atenolol and metoprolol reduced resting cardiac work load. During tyramine-stimulated norepinephrine release, they had little effect on work load, and increased the transient rise in total peripheral vascular resistance, particularly atenolol when combined with losartan. In conclusion, β₁AR, like β₂AR, stimulated norepinephrine but not epinephrine release, independent of adrenal catecholamines, ganglion transmission, or renal renin release/angiotensin AT1 receptor activation. β₁AR therefore functioned as a peripheral, presynaptic, facilitating auto-receptor. Like tyramine, hypoxia may induce NET-mediated release. Augmented tyramine-induced vasoconstriction, as observed after injection of β₁AR-blocker, particularly atenolol combined with losartan, may hamper organ perfusion, and may have clinical relevance in hypoxic conditions such as myocardial infarction.

Influence of Fimasartan (a Novel AT(1) Receptor Blocker) on Catecholamine Release in the Adrenal Medulla of Spontaneously Hypertensive Rats

The aim of this study was to determine whether fimasartan, a newly developed AT₁ receptor blocker, can affect the CA release in the isolated perfused model of the adrenal medulla of spontaneously hypertensive rats (SHRs). Fimasartan (5~50 µM) perfused into an adrenal vein for 90 min produced dose- and time-dependently inhibited the CA secretory responses evoked by ACh (5.32 mM), high K⁺ (56 mM, a direct membrane depolarizer), DMPP (100 µM) and McN-A-343 (100 µM). Fimasartan failed to affect basal CA output. Furthermore, in adrenal glands loaded with fimasartan (15 µM), the CA secretory responses evoked by Bay-K-8644 (10 µM, an activator of L-type Ca²⁺ channels), cyclopiazonic acid (10 µM, an inhibitor of cytoplasmic Ca²⁺-ATPase), and veratridine (100 µM, an activator of Na⁺ channels) as well as by angiotensin II (Ang II, 100 nM), were markedly inhibited. In simultaneous presence of fimasartan (15 µM) and L-NAME (30 µM, an inhibitor of NO synthase), the CA secretory responses evoked by ACh, high K⁺, DMPP, Ang II, Bay-K-8644, and veratridine was not affected in comparison of data obtained from treatment with fimasartan (15 µM) alone. Also there was no difference in NO release between before and after treatment with fimasartan (15 µM). Collectively, these experimental results suggest that fimasartan inhibits the CA secretion evoked by Ang II, and cholinergic stimulation (both nicotininc and muscarinic receptors) as well as by membrane depolarization from the rat adrenal medulla. It seems that this inhibitory effect of fimasartan may be mediated by blocking the influx of both Na⁺ and Ca²⁺ through their ion channels into the rat adrenomedullary chromaffin cells as well as by inhibiting the Ca²⁺ release from the cytoplasmic calcium store, which is relevant to AT₁ receptor blockade without NO release.

Peripheral galanin receptor 2 as a target for the modulation of pain

The neuropeptide galanin is widely expressed in the nervous system and has an important role in nociception. It has been shown that galanin can facilitate and inhibit nociception in a dose-dependent manner, principally through the central nervous system, with enhanced antinociceptive actions after nerve injury. However, following nerve injury, expression of galanin within the peripheral nervous system is dramatically increased up to 120-fold. Despite this striking increase in the peripheral nervous system, few studies have investigated the role that galanin plays in modulating nociception at the primary afferent nociceptor. Here, we summarise the recent work supporting the role of peripherally expressed galanin with particular reference to the dual actions of the galanin receptor 2 in neuropathic pain highlighting this as a potential target analgesic.

Activation of the galanin receptor 2 in the periphery reverses nerve injury-induced allodynia

Background

Galanin is expressed at low levels in the intact sensory neurons of the dorsal root ganglia with a dramatic increase after peripheral nerve injury. The neuropeptide is also expressed in primary afferent terminals in the dorsal horn, spinal inter-neurons and in a number of brain regions known to modulate nociception. Intrathecal administration of galanin modulates sensory responses in a dose-dependent manner with inhibition at high doses. To date it is unclear which of the galanin receptors mediates the anti-nociceptive effects of the neuropeptide and whether their actions are peripherally and/or centrally mediated. In the present study we investigated the effects of direct administration into the receptive field of galanin and the galanin receptor-2/3-agonist Gal2-11 on nociceptive primary afferent mechanical responses in intact rats and mice and in the partial saphenous nerve injury (PSNI) model of neuropathic pain.

Results

Exogenous galanin altered the responses of mechano-nociceptive C-fibre afferents in a dose-dependent manner in both naive and nerve injured animals, with low concentrations facilitating and high concentrations markedly inhibiting mechano-nociceptor activity. Further, use of the galanin fragment Gal2-11 confirmed that the effects of galanin were mediated by activation of galanin receptor-2 (GalR2). The inhibitory effects of peripheral GalR2 activation were further supported by our demonstration that after PSNI, mechano-sensitive nociceptors in galanin over-expressing transgenic mice had significantly higher thresholds than in wild type animals, associated with a marked reduction in spontaneous neuronal firing and C-fibre barrage into the spinal cord.

Conclusions

These findings are consistent with the hypothesis that the high level of endogenous galanin in injured primary afferents activates peripheral GalR2, which leads to an increase in C-fibre mechanical activation thresholds and a marked reduction in evoked and ongoing nociceptive responses.

Inhibitory effects of olmesartan on catecholamine secretion from the perfused rat adrenal medulla

The present sutdy aimed to determine whether olmesartan, an angiotensin II (Ang II) type 1 (AT(1)) receptor blocker, can influence the CA release from the isolated perfused model of the rat adrenal medulla. Olmesartan (5~50 µM) perfused into an adrenal vein for 90 min produced dose- and time-dependent inhibition of the CA secretory responses evoked by ACh (5.32 mM), high K(+) (56 mM, a direct membrane-depolarizer), DMPP (100 µM) and McN-A-343 (100 µM). Olmesartan did not affect basal CA secretion. Also, in adrenal glands loaded with olmesartan (15 µM), the CA secretory responses evoked by Bay-K-8644 (10 µM, an activator of voltage-dependent L-type Ca(2+) channels), cyclopiazonic acid (10 µM, an inhibitor of cytoplasmic Ca(2+) -ATPase), veratridine (100 µM, an activator of voltage-dependent Na(+) channels), and Ang II (100 nM) were markedly inhibited. However, at high concentrations (150~300 µM), olmesartan rather enhanced the ACh-evoked CA secretion. Taken together, these results show that olmesartan at low concentrations inhibits the CA secretion evoked by cholinergic stimulation (both nicotininc and muscarinic receptors) as well as by direct membrane depolarization from the rat adrenal medulla, but at high concentrations it rather potentiates the ACh-evoked CA secretion. It seems that olmesartan has a dual action, acting as both agonist and antagonist at nicotinic receptors of the isolated perfused rat adrenal medulla, which might be dependent on the concentration. It is also thought that this inhibitory effect of olmesartan may be mediated by blocking the influx of both Na(+) and Ca(2+) into the rat adrenomedullary chromaffin cells as well as by inhibiting the Ca(2+) release from the cytoplasmic calcium store, which is thought to be relevant to the AT(1) receptor blockade, in addition to its enhancement on the CA secreton.

Localization of type-2 angiotensin II receptor in adrenal gland

The localization of the type-2 angiotensin II receptor (AT2) in the adrenal glands of rats, guinea pigs, bovines, and humans was examined at the mRNA and protein levels. PCR products for AT2 were detected in the adrenal cortices and adrenal medullae of all the mammals examined with an RT-PCR technique. Three different anti-AT2 antibodies (Abs), whose specificity was confirmed in our hands, recognized a 50-kDa protein in the adrenal glands of the four mammals, and this recognition was abolished by the preabsorption of an Ab with an antigen. Immunoblotting and immunohistochemistry revealed that the 50-kDa protein was expressed consistently and variably in the adrenal cortices and medullae of various mammals, respectively. We conclude that the 50-kDa AT2 is consistently expressed in the adrenal cortex in a wide variety of mammals.

Effects of losartan on catecholamine release in the isolated rat adrenal gland

The aim of this study was to determine whether losartan, an angiotensin II (Ang II) type 1 (AT(1)) receptor could influence the CA release from the isolated perfused model of the rat adrenal medulla. Losartan (5~50 microM) perfused into an adrenal vein for 90 min produced dose- and time-dependent inhibition of the CA secretory responses evoked by ACh (5.32 mM), high K(+) (56 mM, a direct membrane depolarizer), DMPP (100 microM) and McN-A-343 (100 microM). Losartan failed to affect basal CA output. Furthermore, in adrenal glands loaded with losartan (15 microM) for 90 min, the CA secretory responses evoked by Bay-K-8644 (10 microM, an activator of L-type Ca(2+) channels), cyclopiazonic acid (10 microM, an inhibitor of cytoplasmic Ca(2+)-ATPase), veratridine (100 microM, an activator of Na(+) channels), and Ang II (100 nM) were markedly inhibited. However, at high concentrations (150~300 microM), losartan rather enhanced the CA secretion evoked by ACh. Collectively, these experimental results suggest that losartan at low concentrations inhibits the CA secretion evoked by cholinergic stimulation (both nicotininc and muscarinic receptors) as well as by membrane depolarization from the rat adrenal medulla, but at high concentration it rather inhibits ACh-evoked CA secretion. It seems that losartan has a dual action, acting as both agonist and antagonist to nicotinic receptors of the rat adrenal medulla, which might be dependent on the concentration. It is also thought that this inhibitory effect of losartan may be mediated by blocking the influx of both Na(+) and Ca(2+) into the rat adrenomedullary chromaffin cells as well as by inhibiting the Ca(2+) release from the cytoplasmic calcium store, which is thought to be relevant to the AT(1) receptor blockade, in addition to its enhancement of the CA release.

The use of viral gene transfer in studies of brainstem noradrenergic and serotonergic neurons

In contrast to some other neuronal populations, for example hippocampal or cortical pyramidal neurons, mechanisms of synaptic integration and transmitter release in central neurons that contain noradrenaline (NA) and serotonin (5HT) are not well understood. These cells, crucial for a wide range of autonomic and behavioural processes, have long un-myelinated axons with hundreds of varicosities where transmitters are synthesized and released. Both seem to signal mostly in 'volume transmission' mode. Very little is known about the rules that apply to this type of transmission in the brain and the factors that regulate the release of NA and 5HT. We discuss some of our published studies and more recent experiments in which viral vectors were used to investigate the physiology of these neuronal populations. We also focus on currently unresolved issues concerning the mechanism of volume transmission by NA and 5HT in the brain. We suggest that clarifying the role of astroglia in this process could be essential for our understanding of central noradrenergic and 5HT signalling.

Area-specific differences in transmitter release in central catecholaminergic neurons of spontaneously hypertensive rats

The link among blood pressure, sympathetic output, and brain neurons producing catecholamines is well documented. Nevertheless, their intrinsic properties and any alterations in signaling characteristics between normotensive and hypertensive phenotypes remain unknown. Here, we directly compared neurophysiological properties of catecholamine release of C1 and A2 neurons of the spontaneously hypertensive rat and Wistar rat in organotypic slices. C1 and A2 areas were studied because both are widely implicated in the pathophysiology of hypertension. Catecholaminergic neurons were visualized using viral vectors to express green fluorescent protein. Microamperometry revealed that C1 axonal varicosities of spontaneously hypertensive but not normotensive Wistar rats release a transmitter predominantly (approximately 86%) in very large quanta, comparable in catecholamine load to adrenal chromaffin granules. Because quantal size affects the spread of transmitter in the extracellular space, this may enhance the impact of C1 varicosities on their downstream targets and increase sympathetic drive in the hypertensive rat. Electrophysiological properties and Ca2+ handling were studied using patch clamp and confocal imaging. Although overall electrophysiological characteristics of C1 and A2 neurons were comparable between strains, the characteristic angiotensin-II-induced Ca2+ mobilization was reduced in A2 neurons of the spontaneously hypertensive rat. Because A2 neurons are a part of a homeostatic antihypertensive circuit, this could reduce their restraining influence on blood pressure. Thus, we have revealed an increased quantal size in C1 varicosities and a reduced responsiveness of A2 neurons of the spontaneously hypertensive rat to angiotensin II. Both effects could contribute to elevated sympathetic activity and blood pressure in the spontaneously hypertensive rat.

Measuring secretion in chromaffin cells using electrophysiological and electrochemical methods

Our present understanding of exocytosis of catecholamines has benefited tremendously from the arrival of single-cell electrochemical methods (amperometry and voltammetry), electrophysiological techniques (whole-cell and patch capacitance) and from the combination of both techniques (patch amperometry). In this brief review, we will outline the strengths and limitations of amperometric and electrophysiological methods and highlight the major contribution obtained with the use of these techniques in chromaffin cells.

N- and P/Q-type Ca2+ channels in adrenal chromaffin cells

Ca2+ is the most ubiquitous second messenger found in all cells. Alterations in [Ca2+]i contribute to a wide variety of cellular responses including neurotransmitter release, muscle contraction, synaptogenesis and gene expression. Voltage-dependent Ca2+ channels, found in all excitable cells (Hille 1992), mediate the entry of Ca2+ into cells following depolarization. Ca2+ channels are composed of a large pore-forming subunit, called the alpha1 subunit, and several accessory subunits. Ten different alpha1 subunit genes have been identified and classified into three families, Ca(v1-3) (Dunlap et al. 1995, Catterall 2000). Each alpha1 gene produces a unique Ca2+ channel. Although chromaffin cells express several different types of Ca2+ channels, this review will focus on the Cav(2.1) and Cav(2.2) channels, also known as P/Q- and N-type respectively (Nowycky et al. 1985, Llinas et al. 1989b, Wheeler et al. 1994). These channels exhibit physiological and pharmacological properties similar to their neuronal counterparts. N-, P/Q and to a lesser extent R-type Ca2+ channels are known to regulate neurotransmitter release (Hirning et al. 1988, Horne & Kemp 1991, Uchitel et al. 1992, Luebke et al. 1993, Takahashi & Momiyama 1993, Turner et al. 1993, Regehr & Mintz 1994, Wheeler et al. 1994, Wu & Saggau 1994, Waterman 1996, Wright & Angus 1996, Reid et al. 1997). N- and P/Q-type Ca2+ channels are abundant in nerve terminals where they colocalize with synaptic vesicles. Similarly, these channels play a role in neurotransmitter release in chromaffin cells (Garcia et al. 2006). N- and P/Q-type channels are subject to many forms of regulation (Ikeda & Dunlap 1999). This review pays particular attention to the regulation of N- and P/Q-type channels by heterotrimeric G-proteins, interaction with SNARE proteins, and channel inactivation in the context of stimulus-secretion coupling in adrenal chromaffin cells.

Upregulation of AT1 receptor gene on activation of protein kinase Cbeta/nicotinamide adenine dinucleotide diphosphate oxidase/ERK1/2/c-fos signaling cascade mediates long-term pressor effect of angiotensin II in rostral ventrolateral medulla

OBJECTIVE

Angiotensin II induces the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) 1/2 via the activation of nicotinamide adenine dinucleotide diphosphate (NADPH) oxidase on stimulation of the angiotensin subtype 1 receptor (AT1R) in the rostral ventrolateral medulla (RVLM), where sympathetic premotor neurons for the maintenance of vasomotor tone and blood pressure are located. Angiotensin II-activated p38 MAPK in RVLM promotes a short-term pressor effect via augmented glutamatergic neurotransmission. We tested the hypothesis that the NADPH oxidase-dependent phosphorylation of ERK1/2 after the activation of conventional protein kinase C (PKC) mediates the AT1R-dependent long-term pressor effects of angiotensin II via transcriptional induction of the proto-oncogene c-fos gene in RVLM.

METHODS AND RESULTS

In Sprague-Dawley rats, a microinjection of angiotensin II bilaterally into the RVLM induced membrane-bound translocation of the conventional PKCalpha, PKCbeta or PKCgamma isoform, phosphorylation of the p47 subunit of NADPH oxidase and ERK1/2, followed by phosphorylation of the transcription factor cyclic adenosine monophosphate response element binding protein (CREB), and c-fos induction. The PKC inhibitor antagonized angiotensin II-induced p47 phosphorylation, and an antisense oligonucleotide (ASON) complementary to PKCbeta messenger RNA suppressed angiotensin II-induced ERK1/2 activation, phosphorylation or DNA binding activity of CREB, and upregulation of c-fos mRNA expression in the ventrolateral medulla. Furthermore, a microinjection of ERK1/2, CREB or c-fos ASON into the RVLM significantly reduced the long-term pressor effect and augmented AT1R expression in the ventrolateral medulla induced by intracerebroventricular infusion of angiotensin II.

CONCLUSION

We concluded that the PKCbeta/NADPH oxidase/ERK1/2/CREB/c-fos cascade represents a novel signaling cascade that mediates the long-term pressor effect induced by angiotensin II in the RVLM.

Intracellular signaling mechanisms mediating catecholamine release upon activation of NPY Y1 receptors in mouse chromaffin cells

The adrenal chromaffin cells synthesize and release catecholamine (mostly epinephrine and norepinephrine) and different peptides, such as the neuropeptide Y (NPY). NPY stimulates catecholamine release through NPY Y1 receptor in mouse chromaffin cells. The aim of our study was to determine the intracellular signaling events coupled to NPY Y1 receptor activation that lead to stimulation of catecholamine release from mouse chromaffin cells. The stimulatory effect of NPY mediated by NPY Y1 receptor activation was lost in the absence of extracellular Ca2+. On the other hand, inhibition of nitric oxide synthase and guanylyl cyclase also decreased the stimulatory effect of NPY. Moreover, catecholamine release stimulated by NPY or by the nitric oxide donor (NOC-18) was inhibited by mitogen-activated protein kinase (MAPK) and protein kinase C inhibitors. In summary, in mouse chromaffin cells, NPY evokes catecholamine release by the activation the NPY Y1 receptor, in a Ca2+-dependent manner, by activating mitogen-activated protein kinase and promoting nitric oxide production, which in turn regulates protein kinase C and guanylyl cyclase activation.

Angiotensin II increases evoked release at the frog neuromuscular junction through a receptor sensitive to A779

Receptor mediated presynaptic modulation is a ubiquitous mechanism involved in synaptic plasticity. Here we show that angiotensin II increased quantal content at the frog neuromuscular junction. This presynaptic effect of angiotensin II was insensitive to losartan and PD123319, but was antagonized by a more potent partial agonist of the amphibian angiotensin receptor, L162313. In addition, A779, a blocker of the angiotensin-[1-7] receptor, also abolished the effect of angiotensin II. These results indicate that the effect of angiotensin II on evoked release is mediated through an angiotensin receptor. L162313 alone increased quantal content, and A779 also antagonized this effect of L162313. We conclude that the neuromuscular junction possesses angiotensin receptors involved in presynaptic modulation.

Exocytosis of norepinephrine at axon varicosities and neuronal cell bodies in the rat brain

Norepinephrine secretion from central neurons was widely assumed to occur by exocytosis, but the essential characteristics of this process remained unknown. We developed an approach to study it directly by amperometry using carbon fiber microelectrodes in organotypic rat brainstem slice cultures. Noradrenergic neurons from areas A1 and A2 were fluorescently labeled by an adenoviral vector with noradrenergic-specific promoter. Quantal events, consistent with exocytotic release of norepinephrine, were registered at noradrenergic axonal varicosities as well as at cell bodies. According to their charge integrals, events were grouped into two populations. The majority (approximately 40 fC) were compatible with full exocytotic fusion of small clear and dense core vesicles shown in previous morphometric studies. The quantal size distribution was modulated by treatment with reserpine and amitriptyline. In addition, much larger quantal events (>1 pC) occurred at predominantly axonal release sites. The time course of signals was severalfold faster than in adrenal chromaffin cells, suggesting profound differences in the release machinery between these cell types. Tetrodotoxin eliminated the majority of events, indicating that release was partially, but not entirely, action potential driven. In conclusion, central norepinephrine release has unique characteristics, distinguishing it from those of other monoaminergic cells in periphery and brain.

Lysophospholipids elevate [Ca2+]i and trigger exocytosis in bovine chromaffin cells

Sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are responsible for many physiological functions, including angiogenesis, neuronal survival, and immunity. However, little is known about their effects in modulating the stimulus-secretion coupling in bovine chromaffin cells. The result of PCR showed that at least two receptors (S1P(3) and LPA(1)) were expressed in bovine chromaffin cells. The elevation of [Ca(2+)](i) by S1P was fast and sustaining; but the elevation by LPA was slow and transient. The EC(50) for S1P and LPA in elevating the [Ca(2+)](i) were 0.55+/-0.01 and 0.54+/-0.40microM, respectively. This elevation could be totally blocked by thapsigargin, 2-APB, and U73122. Pertussis toxin pretreatment inhibited about half of the elevation in [Ca(2+)](i) suggesting the involvement of G(i) and other G-proteins. Repetitive [Ca(2+)](i) elevations elicited by S1P, but not LPA, were inhibited by ryanodine. S1P was more effective than LPA in triggering exocytosis as measured by the changes in membrane capacitance. The whole-cell Ca(2+) current was inhibited by both lysophospholipids but Na(+) current was inhibited by S1P only. These results suggest the differential effects of LPA and S1P in releasing Ca(2+) from the intracellular Ca(2+) stores and modulating the stimulus-secretion coupling in bovine chromaffin cells.

Real-time measurements of noradrenaline release in periphery and central nervous system

Noradrenaline (NA) plays important hormonal and neurotransmitter roles in the periphery and central nervous system, respectively. The cells that produce and release NA, namely, adrenal chromaffin cells (ACCs), sympathetic postganglionic neurones and central neurones, show both commonalities as well as profound differences in morphology, physiological function and characteristics of NA secretion. In order to address disorders which have been associated with the dysregulation of NA release, such as essential hypertension, a better understanding of the molecular mechanisms governing and modulating NA release in neurones is urgently required. Due to profound technical challenges, the molecular basis of NA release has been investigated much more thoroughly in ACCs than in neurones. This review discusses suitable approaches for detecting NA secretion in periphery as well as brain tissues. Membrane capacitance and high-resolution electrochemical measurements have proven particularly useful when combined with fluorescence microscopy. ACCs and peripheral and central NAergic neurones are compared regarding their vesicle morphologies, as well as possible locations of release sites, and the trajectory of secreted NA. Further, current views on the properties of single vesicle release events, including proposed release probabilities in these cell types, are presented.

Identification of the P2Y(12) receptor in nucleotide inhibition of exocytosis from bovine chromaffin cells

Nucleotides are released from bovine chromaffin cells and take part in a feedback loop to inhibit further exocytosis. To identify the nucleotide receptors involved, we measured the effects of a range of exogenous nucleotides and related antagonists on voltage-operated calcium currents (I(Ca)), intracellular calcium concentration ([Ca(2+)](i)), and membrane capacitance changes. In comparative parallel studies, we also cloned the bovine P2Y(12) receptor from chromaffin cells and determined its properties by coexpression in Xenopus laevis oocytes with inward-rectifier potassium channels made up of Kir3.1 and Kir3.4. In both systems, the agonist order of potency was essentially identical (2-methylthio-ATP approximately 2-methylthio-ADP >> ATP approximately ADP > UDP). alphabeta-Methylene-ATP and adenosine were inactive. UTP inhibited I(Ca) in chromaffin cells (pEC(50) = 4.89 +/- 0.11) but was essentially inactive at the cloned P2Y(12) receptor. The relatively nonselective P2 antagonist pyridoxal-phosphate-6-azophenyl-2',4' disulfonic acid blocked nucleotide responses in both chromaffin cells and X. laevis oocytes, whereas the P2Y(12)- and P2Y(13)-selective antagonist N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene ATP (ARC69931MX) blocked responses to ATP in both chromaffin cells and X. laevis oocytes but not to UTP in chromaffin cells. These results identify the P2Y(12) purine receptor as a key component of the nucleotide inhibitory pathway and also demonstrate the involvement of a UTP-sensitive G(i/o) -coupled pyrimidine receptor.

Viral vectors as tools for studies of central cardiovascular control

During the last few years physiological genomics has been the most rapidly developing area of physiology. Given the current ease of obtaining information about nucleotide sequences found in genomes and the vast amount of readily available clones, one of the most pertinent tasks is to find out about the roles of the individual genes and their families under normal and pathological conditions. Viral gene delivery into the brain is a powerful tool, which can be used to address a wide range of questions posed by physiological genomics including central nervous mechanisms regulating the cardio-vascular system. In this paper, we will give a short overview of current data obtained in this field using viral vectors and then look critically at the technology of viral gene transfer.

Angiotensin blocks substance P release from renal sensory nerves by inhibiting PGE2-mediated activation of cAMP

Activation of renal sensory nerves involves PGE2-mediated release of substance P (SP) via activation of the cAMP-PKA pathway. The PGE2-mediated SP release is suppressed by a low- and enhanced by a high-sodium (Na+) diet, suggesting an inhibitory effect of ANG. We now examined whether ANG II is present in the pelvic wall and inhibits PGE2-mediated SP release by blocking PGE2-mediated increases in cAMP. ANG II levels in renal pelvic tissue were 710 +/- 95 and 260 +/- 30 fmol/g tissue in rats fed a low- and high-Na+ diet, respectively. In a renal pelvic preparation from high-Na+-diet rats, 0.14 microM PGE2 produced an increase in SP release from 7 +/- 1 to 19 +/- 3 pg/min that was blocked by 15 nM ANG II. Treating pelvises with pertussis toxin (PTX) abolished the effects of ANG II. In pelvises from low-Na+ rats, neither basal nor bradykinin-mediated SP release was altered by PGE2. However, the bradykinin-mediated release of SP was enhanced by the permeable cAMP analog CPT-cAMP, from 4 +/- 1 to 11 +/- 2 pg/min, a response similar to that in normal-Na+-diet rats. In vivo, renal pelvic administration of PGE2 enhanced the afferent renal nerve activity (ARNA) response to bradykinin in normal- but not in low-Na+ diet rats. CPT-cAMP produced similar enhancement of the ARNA responses to bradykinin in normal- and low-Na+-diet rats, 1,670 +/- 490 and 1,760 +/- 400%.s (area under the curve of ARNA vs. time). Similarly, the ARNA responses to increases in renal pelvic pressure were similarly enhanced by CPT-cAMP in normal- and low-Na+-diet rats. In conclusion, renal pelvic ANG II modulates the responsiveness of renal sensory nerves by suppressing PGE2-mediated activation of adenylyl cyclase via a PTX-sensitive mechanism.

Angiotensin II mediates catecholamine and neuropeptide Y secretion in human adrenal chromaffin cells through the AT1 receptor

The aim of the present work was to study the effect of angiotensin II (Ang II) on catecholamines and neuropeptide Y (NPY) release in primary cultures of human adrenal chromaffin cells. Ang II stimulates norepinephrine (NE), epinephrine (EP) and NPY release from perifused chromaffin cells by 3-, 2- and 12-fold, respectively. The NPY release is more sustained than that of catecholamines. We found that the receptor-AT(2) agonist, T(2)-(Ang II 4-8)(2) has no effect on NE, EP and NPY release from chromaffin cells. We further showed that Ang II increases intracellular Ca(2+) concentration ([Ca(2+)](i)). The selective AT(1)-receptor antagonist Candesartan blocked [Ca(2+)](i) increase by Ang II, while T(2)-(Ang II 4-8)(2) was ineffective. These findings demonstrate that AT(1) stimulation induces catecholamine secretion from human adrenal chromaffin cells probably by raising cytosolic calcium.

Genetic and pharmacological dissection of pathways involved in the angiotensin II-mediated depression of baroreflex function

Heart failure and hypertension are associated with increases in angiotensin II (ANG II) activity. One brain area where ANG II effects may be particularly important in these situations is the nucleus of the solitary tract (NTS). Located in the dorsomedial medulla, the NTS is the termination site of baroreceptor afferents and is essential for mediating the baroreflex. In hypertensive animals the baroreflex is impaired; this may be reversed by antagonizing ANG II AT1 receptors in the NTS. Recently, we showed that the baroreflex depressant action of ANG II in the NTS is mediated by activation of endothelial nitric oxide synthase (eNOS) and enhanced release of GABA. Using conventional pharmacological tools and a range of adenoviral-mediated expression of dominant negative proteins, we have determined the intracellular pathway(s) in the NTS by which ANG II activates eNOS. Our data indicate that ANG II acting in the NTS depresses the baroreflex via a Gq protein-mediated activation of phospholipase C, which through 1,4,5-inositol triphosphate causes release of calcium from the IP3-sensitive intracellular stores and calcium-calmodulin formation. In contrast, multiple site disruption of a pathway leading to eNOS activation via the serine/threonine kinase Akt was ineffective

TRPC4 can be activated by G-protein-coupled receptors and provides sufficient Ca(2+) to trigger exocytosis in neuroendocrine cells

Transient receptor potential (TRP) channels form a large family of plasma membrane cation channels. Mammalian members of the "short" TRP family (TRP channel (TRPC) 1-7 are Ca(2+)-permeant, non-selective cation channels that are widely expressed in various cell types, including neurons. TRPC activity is linked through unknown mechanisms to G-protein-coupled receptors or receptor tyrosine kinases that activate phospholipase C. To investigate the properties and function of TRPC4 in neuronally derived cells, we transiently expressed mouse TRPC4 and histamine H(1) receptor in mouse adrenal chromaffin cells and PC12 cells. Histamine, but not thapsigargin, stimulated Mn(2+) influx in transfected cells. In the whole-cell patch clamp mode, histamine triggered a transient current in TRPC4-expressing cells. No current was evoked by perfusion with inositol 1,4,5-trisphosphate. When exocytosis was monitored with the capacitance detection technique, the magnitude of the membrane capacitance increase (Delta C(m)) on application of histamine in H(1) receptor/TRPC4-expressing chromaffin cells was comparable with that triggered by a train of depolarizing pulses. Our results indicate that TRPC4 channels behave as receptor, but not store-operated, channels in neuronally derived cells. TRPC4 channels can provide sufficient Ca(2+) influx to trigger a robust secretory response in voltage-clamped neurosecretory cells. Similar mechanisms may modulate exocytosis in other neuronal systems.

Ceramide modulates nicotinic receptor-dependent Ca(2+) signaling in rat chromaffin cells

Ceramide, which is an integral component of the sphingomyelin signaling pathway, can attenuate voltage-gated Ca(2+) channel (VGCC) activity in a number of cell types. The aim of the present study was to determine whether ceramide can also modulate VGCC activity, and as a consequence nicotinic receptor-dependent Ca(2+) signaling and catecholamine secretion, in rat adrenal chromaffin cells. Short-term C(6)-ceramide (CER) treatment dose-dependently inhibited nicotine (NIC)-induced peak intracellular Ca(2+) transients. Sphingomyelinase elicited similar responses, whereas the inactive ceramide analog C(2)-dihydroceramide had no effect on NIC-induced Ca(2+) transients. CER suppressed KCl- and NIC-induced Ca(2+) transients to a similar extent, suggesting that the voltage-gated Ca(2+) channel was a primary site of inhibition. In direct support of this concept, whole-cell patch-clamp analysis demonstrated that CER and sphingomyelinase significantly reduced peak Ca(2+) currents. Pretreatment with staurosporine significantly attenuated CER-dependent inhibition of both NIC-induced Ca(2+) transients and peak Ca(2+) current, suggesting that the effects of CER are mediated at least in part by protein kinase C. Consistent with suppressed Ca(2+) signaling, CER also significantly inhibited NIC-induced catecholamine secretion measured at the single-cell level by carbon fiber amperometry. This effect of CER was also significantly attenuated by pretreatment with staurosporine These data demonstrate that the sphingomyelin signaling pathway can modulate nicotinic receptor-dependent Ca(2+) signaling and catecholamine secretion in rat chromaffin cells.

Angiotensin subtype-2 receptor (AT2 ) negatively regulates subtype-1 receptor (AT1 ) in signal transduction pathways in cultured porcine adrenal medullary chromaffin cells

BACKGROUND

Two distinct types of angiotensin II (AngII) receptors, AT1 and AT2, have been cloned. We have shown previously that stimulation of AT2 reduces intracellular cyclic guanosine monophosphate (cGMP) levels in cultured porcine chromaffin cells in which AT2 is the predominantly expressed receptor. However, it has not been determined whether AT1 or AT2 affects signal transduction pathways involving mitogen-activated protein kinases (MAPKs) and signal transducers and activators of transcription (STATs) in chromaffin cells. Also, it is unclear whether cGMP/protein kinase G (PKG) is involved in the regulation of MAPKs and STATs in these cells.

DESIGN

Chromaffin cells were derived from porcine adrenal medulla. The effects of AngII alone (representing physiological conditions), AngII plus CV-11974 (an AT1 antagonist, which simulates specific AT2 stimulation), AngII plus PD 123319 (an AT2 antagonist, which simulates specific AT1 stimulation), and 8-Br-cGMP (a membrane-permeable cGMP analogue) alone on MAPKs (ERKs, JNK, p-38 MAPK) and STATs (STATs 1, 3 and 5) activity were measured.

METHODS

Phosphorylated MAPKs (extracellular signal-related kinases (ERKs), c-jun N-terminal kinase (JNK) and p38 MAPK) and STATs (STATs 1, 3 and 5) were measured by immunoprecipitation-Western blot analysis (IP-Western blot).

RESULTS

AT1 stimulation markedly increased expression of ERKs, JNK, p38 MAPK via Ca2+-dependent protein kinase C (PKC) isoforms (cPKC), as well as STATs 1, 3 and 5 in cultured porcine chromaffin cells. In contrast, AT2 stimulation markedly decreased the expression of these signaling molecules. Also, 8-Br-cGMP alone induced increases in ERKs, JNK, p38 MAPK, and STATs 1, 3 and 5. Because AT2 inhibits cGMP production, we speculate that AT2 may act to suppress cGMP production, which in turn reduces the activity of both MAPKs and STATs in chromaffin cells.

CONCLUSION

AT2 negatively regulates AT1 in signal transduction pathways in chromaffin cells.

Angiotensin II-induced inhibition of calcium currents in hamster submandibular ganglion neurons

Angiotensin II (Ang II) is one of the most important vasoconstrictive hormones but is also known to act as a neuromodulator and a neurotransmitter in the central and peripheral nervous systems. In a previous study, we have shown that Ang II, via AT1 receptors, induced depolarization by inhibition of M-type K(+) channels and SK channels in submandibular ganglion (SMG) neurons. In this study, we investigated the effects of Ang II on calcium channel current (I(Ca)) in acutely dissociated SMG neurons by the patch-clamp technique using the whole-cell configuration. Ang II inhibited total I(Ca) by 32.1+/-2.7%. The half-maximum inhibitory concentration (IC(50)) of Ang II for inhibiting I(Ca) was 0.8 microM. In the presence of 1 microM losartan, which is a selective antagonist of AT1 receptors, the effect of Ang II was attenuated (7.6+/-1.5%). Application of a strong depolarizing voltage prepulse did not affect the Ang II-induced inhibition of I(Ca) (32.8+/-2.8%). Intracellular dialysis of GDP-beta-S attenuated the inhibition of I(Ca) (6.8+/-2.1%). The mean percentage inhibitions of L-, N- and P/Q-type VDCCs by Ang II were 29.1+/-1.7, 16.3+/-6.0 and 1.2+/-0.8%, respectively, of the total I(Ca).