GTP and Ca2+ modulate the inositol 1,4,5-trisphosphate-dependent Ca2+ release in streptolysin O-permeabilized bovine adrenal chromaffin cells

Inhibitory mechanism of bromocriptine on catecholamine release evoked by cholinergic stimulation and membrane depolarization from the rat adrenal medulla

The purpose of this study was to determine whether bromocriptine affects the catecholamines (CA) secretion evoked in isolated perfused rat adrenal glands, by cholinergic stimulation, membrane depolarization and calcium mobilization, and to establish the mechanism of its action. The perfusion of bromocriptine (1-10 microM) into an adrenal vein, for 60 min, produced relatively dose-dependent inhibition in the secretion of catecholamines (CA) evoked by acetylcholine (ACh, 5.32 mM), DMPP (100 microM for 2 min), McN-A-343 (100 microM for 2 min), cyclopiazonic acid (CPA, 10 microM for 4 min) and Bay-K-8644 (10 microM for 4 min). High K+ (56 mM)-evoked CA release was also inhibited, although not in a dose-dependent fashion. Also, in the presence of apomorphine (100 microM), which is also known to be a selective D2-agonist, the CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid were also significantly depressed. However, in adrenal glands preloaded with bromocriptine (3 microM) in the presence of metoclopramide (15 microM), a selective D2-antagonist, the CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid considerably recovered as compared to that of bromocriptine only. Taken together, these results suggest that bromocriptine can inhibit the CA secretion evoked by stimulation of cholinergic receptors, as well as by membrane depolarization, in the perfused rat adrenal medulla. It is thought this inhibitory effect of bromocriptine may be mediated by inhibiting the influx of extracellular calcium and the release from intracellular calcium stores, through the activation of dopaminergic D2-receptors located in the rat adrenomedullary chromaffin cells. Furthermore, these findings also suggest that the dopaminergic D2-receptors may play an important role in regulating adrenomedullary CA secretion.

Inhibitory mechanism of pinacidil on catecholamine secretion from the rat perfused adrenal gland evoked by cholinergic stimulation and membrane depolarization

1. The present study attempted to investigate the effect of potassium channel openers on secretion of catecholamines (CA) evoked by cholinergic stimulation and membrane depolarization from rat isolated perfused adrenal gland. 2. The perfusion of pinacidil (30-300 microM) into an adrenal vein for 20 min produced dose-dependent inhibition of CA secretion evoked by acetylcholine (ACh; 5.32 mM), high K+ (56 mM), 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP; 100 microM for 2 min), 3-(m-chloro-phenyl-carbamoyl-oxy)-2-butynyl trimethyl ammonium chloride (McN-A-343; 100 microM for 2 min), cyclopiazonic acid (CPA; 10 microM for 4 min) and methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyri dine-5-carboxylate (Bay-K-8644; 10 microM for 4 min). 3. In the presence of minoxidil (100 microM), which is also known to be a potassium channel activator, CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and CPA were also significantly depressed. 4. In adrenal glands preloaded with pinacidil (100 microM) in the presence of glibenclamide (GB; 1 microM), a specific blocker of ATP-regulated potassium channels, CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and CPA were restored to a considerable extent of the control release as compared with that of pinacidil only. 5. These results suggest that pinacidil causes marked inhibition of CA secretion evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors, as well as by membrane depolarization, indicating that this effect may be mediated by inhibiting influx of extracellular calcium and release of intracellular calcium in the rat adrenomedullary chromaffin cells. Furthermore, these findings suggest that these potassium channel opener-sensitive membrane potassium channels also play a modulatory role in regulating CA secretion.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Ca2+ inhibition of inositol trisphosphate-induced Ca2+ release in single smooth muscle cells of guinea-pig small intestine

1. Single smooth muscle cells from the longitudinal muscle layer of guinea-pig small intestine were voltage clamped using patch pipettes in the whole-cell mode. 2. When D-myo-inositol 1,4,5-trisphosphate (InsP3) was released at intervals, by photolysis of 'caged' InsP3 within the cell, increases in [Ca2+]i in many cells, as judged from Ca(2+)-activated K(+)-current, were all-or-none; release of InsP3 before a critical interval had elapsed, which was quite stable for an individual cell, resulted in no response. After Ca(2+)-induced Ca2+ release had been evoked by depolarization, the InsP3 response was inhibited. Oscillations in [Ca2+]i evoked by muscarinic receptor activation were unaffected by Ruthenium Red; during these oscillations exogenous InsP3 was not effective close to, or shortly after, peak [Ca2+]i but was effective at other times. 3. Reproducible release of Ca2+ and elevation of [Ca2+]i could be produced by brief (up to 0.5 s) pressure applications of 10 mM caffeine at intervals of 10 s or greater but caffeine itself rarely evoked oscillations in [Ca2+]i. Responses to flash release of InsP3 were reduced after caffeine-induced responses and recovery of caffeine-induced Ca2+ release was faster than recovery of InsP3-induced Ca2+ release. 4. The results support the idea that InsP3-induced Ca(2+)-store release can be inhibited by a certain level of [Ca2+]i at a time when Ca2+ stores have refilled and can be released by caffeine; they also support the suggestion that during oscillations of [Ca2+]i evoked by muscarinic receptor activation, Ca2+ inhibition of InsP3-induced Ca2+ release at some critical level of [Ca2+]i allows Ca2+ stores to refill and leads to a fall in [Ca2+]i so contributing to the oscillations which are observed.

Calcium ion homeostasis in smooth muscle

Ca2+ plays an important role in the regulation of smooth-muscle contraction. In this review, we will focus on the various Ca(2+)-transport processes that contribute to the cytosolic Ca2+ concentration. Mainly the functional aspects will be covered. The smooth-muscle inositol 1,4,5-trisphosphate receptor and ryanodine receptor will be extensively discussed. Smooth-muscle contraction also depends on extracellular Ca2+ and both voltage- and Ca(2+)-release-activated plasma-membrane Ca2+ channels will be reviewed. We will finally discuss some functional properties of the Ca2+ pumps that remove Ca2+ from the cytoplasm and of the Ca2+ regulation of the nucleus.

Two roles of Ca2+ in agonist stimulated Ca2+ oscillations

We propose a mechanism for agonist-stimulated Ca2+ oscillations that involves two roles for cytosolic Ca2+: (a) inhibition of inositol-1,4,5-trisphosphate (IP3) stimulated Ca2+ release from the endoplasmic reticulum (ER) and (b) stimulation of the production of IP3 through its action on phospholipase C (PLC), via a Gq protein related mechanism. Relying on quantitative experiments by Parker, I., and I. Ivorra (1990. Proc. Natl. Acad. Sci. USA. 87:260-264) on the inhibition of Ca2+ release from the ER using caged-IP3, we develop a kinetic model of inhibition that allows us to simulate closely their experiments. The model assumes that the ER IP3 receptor is a tetramer of independent subunits that can bind both Ca2+ and IP3. Upon incorporation of the action of Ca2+ on PLC that leads to production of IP3, we observe in-phase-oscillations of Ca2+ and IP3 at intermediate values of agonist stimulation. The oscillations occur on a time scale of 10-20 s, which is comparable to the time scale for inhibition in Xenopus oocytes. Analysis of the mechanism shows that Ca(2+)-inhibition of IP3-stimulated Ca2+ release from the ER is an essential step in the mechanism. We also find that the effect of Ca2+ on PLC can lead to an indirect increase of cytosolic Ca2+, superficially resembling "Ca(2+)-induced Ca(2+)-release." The mechanism that we propose appears to be consistent with recent experiments on REF52 cells by Harootunian, A. T., J. P. Y. Kao, S. Paranjape, and R. Y. Tsien. (1991. Science [Wash. DC]. 251:75-78.) and we propose additional experiments to help test its underlying assumptions.

Decavanadate displaces inositol 1,4,5-trisphosphate (IP3) from its receptor and inhibits IP3 induced Ca2+ release in permeabilized pancreatic acinar cells

Inositol 1,4,5-trisphosphate (IP3) induced Ca2+ release in digitonin permeabilized rat pancreatic acinar cells is specifically inhibited by decavanadate. The Ca2+ release induced with 0.18 microM IP3 is half maximally inhibited with approximately 5 microM decavanadate. Complete inhibition is achieved with around 20 microM decavanadate. Removal of decavanadate from the permeabilized cells fully restores sensitivity towards IP3, indicating the reversibility of the inhibition. Oligovanadate, which inhibits ATP dependent Ca2+ uptake into intracellular stores, does not influence IP3 induced Ca2+ release. In order to reveal the mechanism underlying the effects of the different vanadate species, binding of IP3 to the same cellular preparations was investigated. We found that binding of IP3 to a high affinity receptor site (Kd approx. 1.2 nM) could be abolished by decavanadate but not by oligovanadate. With 0.5 microM decavanadate, IP3 binding was half maximally inhibited. A similar potency of decavanadate was also found with adrenal cortex microsomes which bind IP3 with the same affinity (Kd approx. 1.4 nM) as permeabilized pancreatic acinar cells. Labelled IP3 was displaced from these subcellular membranes with similar kinetics by unlabelled IP3 and decavanadate. The data suggest that the inhibitory action of decavanadate on IP3 induced Ca2+ release is a consequence of its effect on binding of IP3 to its receptor.