Inhibition by omega-conotoxin GVIA of adrenal catecholamine release in response to endogenous and exogenous acetylcholine

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.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

Neuronal Ca2+ Channel Blocking Action of an Antihypertensive Drug, Cilnidipine, in IMR-32 Human Neuroblastoma Cells

Although the anti-sympathetic mechanisms of the antihypertensive drug cilnidipine have been analyzed in neuronal cells derived from rodents, there is little information regarding the effects of cilnidipine in human neuronal cells. We investigated the effects of cilnidipine on N-type Ca2+ channels in IMR-32 human neuroblastoma cells using fura-2-based microfluorimetry. The ratio of the intensities of the emitted fluorescence at an excitation wavelength of 340 nm to that at 380 nm was calibrated to estimate the intracellular concentration of Ca2+. Stimulation of IMR-32 cells by 40 mmol/l KCl immediately increased the intensities ratio. In the presence of 10 micromol/l of nifedipine to block L-type Ca2+ channels, omega-conotoxin GVIA, a selective N-type Ca2+ channel blocker, in a concentration of 1 micromol/l suppressed the elevation of the intensities ratio induced by 40 mmol/l KCl. Similarly, cilnidipine in a concentration of 10 micromo/l suppressed the elevation of the ratio induced by 40 mmol/l KCl, and this suppression was effectively inhibited after the treatment with omega-conotoxin GVIA. These results suggest that cilnidipine potentially inhibits N-type Ca2+ channels in human neuronal cells and might be applied as a prospective therapeutic tool to provide neuronal protection as well as its antihypertensive effects and anti-sympathetic actions.

Effect of cilnidipine, a novel dihydropyridine Ca2+ channel blocker, on adrenal catecholamine secretion in anesthetized dogs

We investigated the effect of cilnidipine, a novel dihydropyridine Ca2+ channel blocker possessing blocking actions on N-type and L-type voltage-dependent Ca2+ channels (VDCCs), in comparison with the L-type VDCC blocker nifedipine, on adrenal catecholamine secretion in response to splanchnic nerve stimulation (SNS), acetylcholine (ACh), the nicotinic receptor stimulant 1,1-dimethyl-4-phenyl-piperazinium (DMPP), and muscarine in anesthetized dogs. Ca2+ channel blockers and cholinergic agonists were infused and injected, respectively, into the adrenal gland through the phrenicoabdominal artery. Cilnidipine (0.3-3 microg/min) inhibited increases in both epinephrine (EPI) and norepinephrine (NE) output induced by SNS (2 Hz), ACh (1.5 microg), and DMPP (0.2 microg). However, cilnidipine inhibited increase in NE output induced by muscarine (1 microg) without affecting increase in EPI output. Nifedipine (0.3-3 microg/min) inhibited the ACh- and DMPP-induced increases in EPI and NE output without affecting the SNS- and muscarine-induced increases in EPI and NE output. From these results, it seems likely that the inhibition by cilnidipine of the SNS-induced EPI and NE secretion and of the muscarine-induced NE secretion is related to its blocking action on N-type VDCCs.

Effects of a novel antihypertensive drug, cilnidipine, on catecholamine secretion from differentiated PC12 cells

Effects of a novel dihydropyridine type of antihypertensive drug, cilnidipine, on the regulation of the catecholamine secretion closely linked to the intracellular Ca2+ were examined using nerve growth factor (NGF)-differentiated rat pheochromocytoma PC12 cells. By measuring catecholamine secretion with high-performance liquid chromatography coupled with an electrochemical detector, we showed that high K+ stimulation evoked dopamine release from PC12 cells both before and after NGF treatments. Cilnidipine depressed dopamine release both from NGF-treated and untreated PC12 cells in a concentration-dependent manner. In contrast, inhibition by nifedipine was markedly decreased in the differentiated PC12 cells. With intracellular Ca2+ concentration ([Ca2+]i) measurements using fura 2, the elevation of high K+-evoked [Ca2+]i was separated into nifedipine-sensitive and -resistant components. The nifedipine-resistant [Ca2+]i increase was also blocked by cilnidipine, as well as omega-conotoxin-GVIA. By the use of the conventional whole-cell patch-clamp technique, the compositions of the high-voltage-activated Ca2+ channel currents in the NGF-treated PC12 cells were divided into types: L-type, N-type, and residual current components. It was also estimated that cilnidipine at 1 and 3 micromol/L strongly blocked the N-type current without affecting the residual current. These results suggest that cilnidipine inhibits catecholamine secretion from differentiated PC12 cells by blocking Ca2+ influx through the N-type Ca2+ channel, in addition to its well-known action on the L-type Ca2+ channel.

Effects of N- and L-type calcium channel antagonists and (+/-)-Bay K8644 on nerve-induced catecholamine secretion from bovine perfused adrenal glands

1. The effects of N- and L-type calcium channel antagonists and (+/-)-Bay K8644 on catecholamine release from chromaffin cells and acetylcholine release from splanchnic nerve terminals was investigated in bovine perfused adrenal glands. 2. Adrenal glands were perfused retrogradely and preloaded with [3H]-choline. Subsequent efflux of 3H-labelled compounds was taken as an index of acetylcholine release from the splanchnic nerve terminals. Noradrenaline and adrenaline release from the glands was measured by h.p.l.c. with electrochemical detection. 3. A maximally effective frequency of field stimulation of the adrenal nerves, 10 Hz, induced release of catecholamines and 3H-labelled compounds. Tetrodotoxin (1 microM) abolished release of both catecholamines and 3H-labelled compounds. A combination of mecamylamine (5 microM) and atropine (1 microM) inhibited nerve-induced catecholamine release by about 75% but did not inhibit release of 3H-labelled compounds. Reducing the concentration of extracellular calcium 5 fold to 0.5 mM inhibited nerve-induced catecholamine release by 80% and release of 3H-labelled compounds by 50%. 4. (+/-)-Bay K8644 (1 microM), nitrendipine (1 microM), omega-conotoxin-GVIA (10 nM) and the combination of nitrendipine and omega-conotoxin-GVIA each had no effect on nerve-induced release of 3H-labelled compounds. 5. (+/-)-Bay K8644 (1 microM) potentiated nerve-induced catecholamine release by 75%. Nitrendipine (1 microM) reduced release by 20% but this did not reach statistical significance, omega-Conotoxin-GVIA (10 nM) reduced nerve-induced catecholamine release by 75%, while the combination of omega-conotoxin-GVIA and nitrendipine reduced release to the same extent as omega-conotoxin-GVIA alone. 6. Exogenous acetylcholine perfusion through the glands produced a concentration-dependent increase in catecholamine release. The maximally effective concentration of acetylcholine for catecholamine release was > or = 300 microM, while 30 microM acetylcholine gave comparable catecholamine release to that obtained with 10 Hz field stimulation. 7. (+/-)-Bay K8644 (1 microM), nitrendipine (1 microM) and omega-conotoxin-GVIA (10 nM) each had no significant effect on catecholamine release evoked by perfusion of the gland with either a near maximally effective concentration of acetylcholine, 100 microM, or with the lower concentration of 30 microM. 8. The results show that the omega-conotoxin-GVIA-sensitive N-type voltage-sensitive calcium channels located on the chromaffin cells are largely responsible for catecholamine release induced by nerve stimulation in bovine adrenal glands. In contrast, N-type calcium channels are not involved in catecholamine release induced by exogenous acetylcholine. L-type voltage sensitive calcium channels do not play a major role in nerve-induced or exogenously applied acetylcholine-induced catecholamine release. However, the L-type calcium channels do have the potential to augment powerfully nerve-induced catecholamine release. N- and L-type calcium channels do not play a major role in the presynaptic release of acetylcholine.