(+)-PN200-110 and ouabain binding sites in purified bovine adrenomedullary plasma membranes and chromaffin cells

Up-regulation of Ca(2+) channels in vas deferens after chronic treatment of newborn rats with nifedipine

Radioligand binding and contraction techniques were used to verify if L-type Ca(2+) channels are modified in rat vas deferens after treatment with the blocker nifedipine (15 microg), injected at 7, 14, 21 and 28 days after birth. Vas deferens tissue was used 10, 30 and 90 days after the last injection, to verify if modifications are persistent. Binding studies with cell membranes, using [(3)H]isradipine, showed an increase of the density (B(max)) of Ca(2+) channels by more than 60%, after 10 and 30 days, without changes of affinity (K(d)). Maximal contractions (E(max)) of KCl, were increased by 106% and 37%, respectively, after 10 and 30 days, without changes of apparent affinity (pD(2)). After 90 days, the values of B(max), K(d), E(max) and pD(2) were not different from the controls. Differences were also not found for rats injected when adult. It is concluded that treatment of newborn, but not of adult, rats with nifedipine produced a long-lasting, though reversible, up-regulation of L-type Ca(2+) channels.

The actions of ouabain and lithium chloride on cytosolic Ca2+ in single chromaffin cells

The effects of ouabain, Li+ and veratridine on the concentration of cytosolic free Ca2+ ([Ca2+]i) were studied in single fura-2-loaded bovine adrenal chromaffin cells. Superfusion of cells with ouabain (10 microM for 60 min) caused only a delayed mild increase of the Ca2+]i, from around 0.1 microM to 0.2-0.3 microM; this increase was Nao(+)-dependent. Replacement of all NaCl of the Krebs-Hepes solution by LiCl (144 mM) produced a gradual increase of [Ca2+]i, which remained elevated at a stable plateau of 0.4-0.5 microM for 40-50 min. When ouabain (in the presence of normal Nao+) or Li+ (in the absence of Nao+) was given in Krebs-Hepes solution containing no Ca2+, the reintroduction of 2.5 mM Ca2+ produced a fast elevation of the [Ca2+]i. In the case of ouabain-treated cells, the [Ca2+]i curve exhibited an initial phasic component which inactivated to a tonic component. omega-Conotoxin MVIIC (3 microM) and R56865 (10 microM) inhibited the phasic but not the tonic component. Veratridine (30 microM) induced large [Ca2+]i oscillations. Both ouabain or Li+ abolished such oscillations. These results are compatible with ouabain causing elevation of [Ca2+]i in bovine chromaffin cells through a dual mechanism, i.e. cell depolarisation and slowing down of the Na(+)-Ca2+ exchanger of their plasmalemma. Through its binding to the Na+ site on the Na(+)-Ca2+ exchanger, Li+ ions generate powerful Cai2+ signals that might be relevant to its known effects on neurosecretory mechanisms.

Density of apamin-sensitive Ca(2+)-dependent K+ channels in bovine chromaffin cells: relevance to secretion

Three objectives were defined when planning this study: (i) to identify binding sites for [125I]-apamin in intact bovine adrenal medulla chromaffin cells and to estimate their density and selectivity; (ii) to determine whether apamin modified the release of catecholamines evoked by brief pulses of dimethylphenylpiperazinium (DMPP, 1 or 5 microM for 10 sec), histamine (10 microM for 10 sec) or high K+ (20, 35 or 70 mM for 10 sec) applied to superfused cells; and (iii) to test whether apamin affected the profiles of the changes in cytosolic Ca2+ concentrations [Ca2+]i obtained in suspensions of cells loaded with fura-2 and stimulated with DMPP or histamine. At equilibrium, increasing concentrations of [125I]-apamin gave a saturation curve whose Scatchard transformation produced a Kd of 132 pM and a Bmax of 0.72 fmol/10(6) cells. Quinine, tetraethylammonium, charybdotoxin or glibenclamide (blockers of various subtypes of K+ channels) did not inhibit [125I]apamin binding. Binding was blocked by apamin and by d-tubocurarine, two blockers of small-conductance Ca(2+)-activated K+ channels (SK channels). The number of binding sites for [125I]apamin amounted to approx. 900 per single chromaffin cell, 0.72 sites per micron 2 surface area. Apamin (1 microM) enhanced the secretory response to histamine (10 microM), DMPP (1 or 5 microM) and high K+ (20 or 35 mM) by 2-3-fold. The response to 70 mM K+, however, was unaffected. Apamin also enhanced the peak [Ca2+]i increase produced by DMPP or histamine by approx. 30%. Overall, these results strongly support the hypothesis that under physiological conditions, SK channels control some of the electrical activity of chromaffin cells and indirectly, the opening of voltage-dependent Ca2+ channels, the access of Ca2+ to the secretory machinery and the rate of catecholamine release to the circulation from the intact adrenal gland.

Interactions between Ca2+, PCA50941 and Bay K 8644 in bovine chromaffin cells

We describe here the effects of PCA50941 (a novel 1,4-dihydropyridine derivative) comparatively with Bay K 8644 on various parameters in bovine adrenal chromaffin cells. The binding of [3H](+)-isradipine to bovine adrenal medulla plasma membranes was inhibited similarly by PCA50941 and Bay K 8644 at various [Ca2+]o suggesting a common binding site for both compounds on the dihydropyridine receptor. In voltage-clamped chromaffin cells PCA50941 (1 microM) and Bay K 8644 (1 microM) shifted the I-V relationship of whole-cell Ca2+ currents by about 5-10 mV towards more hyperpolarizing potentials. At -20 mV, PCA50941 enhanced ICa by 195 +/- 16% and Bay K 8644 by 288 +/- 51%. Stimulation of fura 2-loaded chromaffin cell suspensions with 17.7 K+/0.5 Ca2+ increased 3-fold the basal [Ca2+]i. PCA50941 increased further the K(+)-evoked peak to 655 nM, and Bay K 8644 to 1129 nM. In the presence of 5 mM Ca2+, PCA50941 or Bay K 8644 increased the [Ca2+] peaks to 427 and 350 nM, respectively. PCA50941 potentiated the release of catecholamines from perfused bovine adrenal glands evoked by 30 s pulses of 17.7 mM K+ in a manner dependent on the [Ca2+]o. Thus at 1, 2.5, 5 and 10 mM Ca2+, secretion was 2.3-, 3.8-, 5- and 4-fold greater than in control glands. Bay K 8644 enhanced the K(+)-induced response 3- and 9-fold at [Ca2+]o of 0.25 or 0.5 mM, respectively; at higher [Ca2+]o the potentiation was similar to that of PCA50941.(ABSTRACT TRUNCATED AT 250 WORDS)

Localized L-type calcium channels control exocytosis in cat chromaffin cells

Depolarizing 1-s pulses to 0 mV from a holding potential of -70 mV, induced whole-cell currents through Ca2+ channels (ICa) in patch-clamped cat adrenal medulla chromaffin cells. The dihydropyridine (DHP) furnidipine (3 microM) reduced the peak current by 47% and the late current by 80%. omega-Conotoxin GVIA (CgTx, 1 microM) reduced the peak ICa by 42% and the late ICa by 55%. Pulses (10 s duration) with 70 mM K+/2.5 mM Ca2+ solution (70 K+/2.5 Ca2+), applied to single fura-2-loaded cat chromaffin cells increased the cytosolic Ca2+ concentration ([Ca2+]i) from 0.1 to 2.21 microM; this increase was reduced by 43.7% by furnidipine and by 42.5% by CgTx. In the perfused cat adrenal gland, secretion evoked by 10-s pulses of 70 K+/2.5 Ca2+ was reduced by 25% by CgTx and by 96% by furnidipine. Similar results were obtained when secretion from superfused isolated cat adrenal chromaffin cells was studied and when using a tenfold lower [Ca2+]o. The results are compatible with the existence of DHP-sensitive (L-type) as well as CgTx-sensitive (N-type) voltage-dependent Ca2+ channels in cat chromaffin cells. It seems, however, that though extracellular Ca2+ entry through both channel types leads to similar increments of averaged [Ca2+]i, the control of catecholamine release is dominated only by Ca2+ entering through L-type Ca2+ channels. This supports the idea of a preferential segregation of L-type Ca2+ channels to localized "hot spots" in the plasmalemma of chromaffin cells where exocytosis occurs.

R56865 inhibits catecholamine release from bovine chromaffin cells by blocking calcium channels

1. The effects of R56865 (a new class of cardioprotective agent which prevents Na+ and Ca2+ overload in cardiac myocytes) on catecholamine release, whole-cell current through Ca2+ channels (IBa) and cytosolic Ca2+ concentrations, [Ca2+]i, have been studied in bovine chromaffin cells. 2. R56865 caused a time- and concentration-dependent blockade of catecholamine release from superfused cells stimulated intermittently with 5 s pulses of 59 mM K+. After 5 min superfusion, a 3 microM concentration inhibited secretion by 20%; the blockade increased gradually with perfusion time, to reach 85% after 40 min. The IC50 to block secretion after 5 min periods of exposure to increasing concentrations of R56865 was around 3.1 microM. The blocking effects of R56865 were reversible after 5-15 min wash out. In high Ca2+ solution (10 mM Ca2+), the degree of blockade of secretion diminished by 20% in comparison with 1 mM Ca2+. 3. In electroporated cells, R56865 (10 microM) did not modify the secretory response induced by the application of 10 microM free Ca2+. 4. R56865 blocked the peak IBa current in a concentration- and time-dependent manner; its IC50 was very similar to that obtained for secretion (3 microM). The compound not only reduced the size of the peak current but also promoted its inactivation; when the effects of R56865 were measured at the most inactivated part of the current, its IC50 was 1 microM. Both the inactivation and the reduction of the peak currents were reversible upon washing out the drug. 5. In fura-2-loaded single chromaffin cells the basal [Ca2+]i of around 100 nM was elevated to a peak of1.5 microM by the application of a 5 s pulse of 59 mM K+. R56865 (10 microM) did not affect the basal [Ca2+]but blocked by 90% the K+-evoked increase. This effect was fully reversible after 5-10 min of wash out.6. The results are compatible with the idea that R56865 blocks Ca2+ entry into K+-depolarized chromaffin cells by promoting the inactivation of voltage-dependent Ca2+ channels; this would lead to the limitation of the rise in [Ca2+]i and of the release of catecholamines. The restriction of catecholamine release may favour indirectly the known direct beneficial cardioprotective actions of R56865.

Separation between cytosolic calcium and secretion in chromaffin cells superfused with calcium ramps

This paper describes experiments in which cytosolic Ca2+ concentrations ([Ca2+]i) and catecholamine release were measured in two populations of chromaffin cells stimulated with a solution enriched in K+ (100 mM). Once depolarized, external Ca2+ or Ba2+ ions were offered to cells either as a single 2.5 mM step or as a ramp that linearly increased the concentration from 0 to 2.5 mM over a 10-min period. A clear separation between the changes of the [Ca2+]i and the time course of secretion was observed. Specifically, secretion and [Ca2+]i rose in parallel when a Ca2+ step was used to reach a peak in a few seconds; however, while secretion declined to the basal level, [Ca2+]i remained elevated at a plateau of 400 nM. With a Ca2+ ramp, only a transient small peak of secretion was observed, yet the [Ca2+]i remained elevated throughout the 10-min stimulation period. The separation between secretion and [Ca2+]i was observed even when voltage-dependent Ca2+ channels were expected to remain open (mild depolarization in the presence of 1 microM Bay K 8644). By using Ba2+ steps or ramps, sustained noninactivating secretory responses were obtained. The results suggest that the rate and extent of secretion are not a simple function of the [Ca2+]i at a given time; they are compatible with the following conclusions: (i) A steep extracellular-to-cytosolic Ca2+ gradient is required to produce a sharp increase in the [Ca2+]i at exocytotic sites capable of evoking a fast but transient secretory response. (ii) As a result of Cai(2+)-dependent inactivation of Ca2+ channels, those high [Ca2+]i are possible only at early times after cell depolarization. (iii) The Cai(2+)-dependent supply of storage granules to the secretory machinery cooperates with the supply of Ca2+ through Ca2+ channels to regulate the rate and extent of secretion.

Low dihydropyridine receptor density in vasa deferentia of castrated rats

Radioligand binding studies in crude membrane preparations of vasa deferentia of normal rats, with the 1,4-dihydropyridine (+)-[3H]-PN200-110 (isradipine) showed typical saturation isotherms. The binding exhibited a KD of 259 +/- 60 pM and Bmax of 144 +/- 20 fmol mg-1 protein. The low KD and the stereoselective displacement of (+)-[3H]-PN200-110 binding by (+)- and (-)-PN200-110 and by nifedipine suggests that these tissues contain dihydropyridine receptors probably coupled to voltage-sensitive, L-type calcium channels. In membrane preparations from vasa deferentia from rats castrated 30 days previously the maximum specific binding was 25 +/- 10 fmol mg-1 protein, representing only 11% of total binding; thus, the calculation of reliable KD values was not feasible. These findings suggest that a testicular hormone, possibly testosterone, plays an important role in the regulation of dihydropyridine-sensitive, voltage-dependent calcium channels in the rat vas deferens.

Separation of two pathways for calcium entry into chromaffin cells

1. The effects of various drugs on 45Ca + 40Ca uptake into cultured bovine adrenal chromaffin cells evoked by 1,1-dimethyl-4-phenylpiperazinium (DMPP) or high K, were studied. In the presence of 1 mM external 40Ca, with 45Ca as a radiotracer, unstimulated cells took up an average of 0.13 fmol/cell 40Ca and 772 c.p.m./10(6) cells of 45Ca (n = 76). Upon stimulation with DMPP (100 microM for 60 s) or K (59 mM for 60 s), Ca uptake increased to 0.92 and 1 fmol/cell, respectively. 2. Flunarizine behaved as a potent blocker of both DMPP- and K-evoked Ca uptake (IC50 of 1.76 and 1.49 microM, respectively for DMPP and K). A similar picture emerged with Cd ions, though Cd exhibited an IC50 against K (1.86 microM) slightly lower than the IC50 against DMPP (8.14 microM). 3. Clear cut differences were observed with amiloride, guanethidine, nimodipine and nisoldipine which behaved as selective blockers of DMPP-mediated Ca uptake responses: IC50 values to block DMPP effects were 290, 27, 1.1 and 1.63 microM respectively for amiloride, guanethidine, nimodipine and nisoldipine. Amiloride blocked K-evoked Ca uptake by only 35% and guanethidine did not affect it. Nisoldipine inhibited K-evoked Ca uptake only partially at low concentrations (about 30%); a second blocking component was observed at the highest concentration used (10 microM). At 10 microM, nimodipine blocked K-evoked Ca uptake by 50%. 4. Thus, it seems that the nicotinic receptor mediated Ca uptake pathway can be pharmacologically separated from the K-activated pathway. The results are compatible with the hypothesis that in cultured bovine adrenal chromaffin cells, stimulation of nicotinic receptors recruits a single type of Ca channel which is sensitive to flunarizine, Cd, amiloride, guanethidine, nimodipine and nisoldipine. The results also suggest that K depolarization might be recruiting in addition to this channel, another Ca channel which is highly sensitive to Cd and flunarizine, resistant to nisoldipine, nimodipine and amiloride, and insensitive to guanethidine.

(+)-isradipine but not (-)-Bay-K-8644 exhibits voltage-dependent effects on cat adrenal catecholamine release

1. Catecholamine release from cat adrenal glands perfused at a high rate (4 ml min-1) at 37 degrees C with polarizing (1.2 or 5.9 mM K+) or depolarizing (17.7, 35, 59 or 118 mM K+) solutions, was triggered by 5 or 10 s pulses of Ca2+ (0.5 or 2.5 mM) in the presence of various concentrations of K+. 2. In polarized glands, secretion was greater the higher the K+ concentration present during the secretory K+/Ca2+ test pulse. Thus, in 17.7 mM K+, catecholamine released was 162 +/- 27 ng per pulse, while in 118 mM K+ secretion rose to 1839 +/- 98 ng per pulse. In depolarized glands, secretion reached a peak of around 1000 ng per pulse in 35-59 mM K+; in 118 mM K+, secretion did not increase further, suggesting that voltage changes are implicated in the control of the secretory process. 3. Blockade of secretion by increased concentrations of (+)-isradipine was much more manifest in polarized glands. The higher the degree of depolarization was (35, 59 or 118 mM K+), the lower the IC50 s were. So, the ratios between the IC50 s in polarized and depolarized glands rose from 3.92 in 35 mM K+ to 26.7 in 118 mM K+. 4. In contrast, the Ca2+ channel activator (-)-Bay K 8644 potentiated catecholamine release evoked by K+/Ca2+ pulses equally well in polarized or depolarized glands. The ratios between EC50 s in polarized or depolarized glands were, respectively, 0.30, 0.59 and 0.69 for 17.7, 35 and 118 mM K+. 5. In simultaneous experiments, the two enantiomers of Bay K 8644 exhibited opposite effects on secretion. (+)-Bay K 8644 (a Ca21 channel blocker) inhibited secretion better in depolarized than in polarized glands, whilst (-)-Bay K 8644 potentiated secretion in a voltage-independent manner. 6. Potentiation of secretion by (-)-Bay K 8644 was concentration-dependent from 10-8 to 10-6M. At 10- 5M, such potentiation largely disappeared in both polarized and depolarized glands. However, this dual effect of (-)Bay K 8644 was better seen in depolarizing conditions, suggesting that using the same enantiomer, the voltage-dependence is only seen when blockade of secretion dominates. 7. In the presence of increasing concentrations of (-)Bay K 8644 (3 x 10-9, 3 x 10-8 and 3 x 10-7M), the concentration-response curves for (+)isradipine to inhibit secretion were displaced to the right. However, a Schild plot of (dose ratio - 1) against (-)-Bay K 8644 concentrations gave a slope of 0.6, suggesting that the interactions between (+)-isradipine and (-)Bay K 8644 were non-competitive in nature. The pA2 for (-)-Bay K 8644 was 9.13. 8. Overall, the results suggest that potentiation of secretion by (-)Bay K 8644 (a voltage-independent phenomenon), and blockade by (+)-isradipine or (+-Bay K 8644 (a voltage-dependent phenomenon) might be exerted through binding of the dihydropyridines activators and blockers to separate sites on chromaffin cell L-type Ca2 + channels.

Mechanism of blockade by (+)isradipine of adrenal catecholamine release

Cat adrenal glands were perfused at a high rate with various modified Krebs solutions containing different concentrations of K+ but no Ca2+. Catecholamine release was tested by applying brief Ca2+ pulses (10 s of a solution containing 120 mM K+ and 2.5 mM Ca2+). Under polarizing conditions (10 min perfusion with 1.4 mM K+ with no Ca2+), the total catecholamines released by the Ca2+ pulse amounted to 5 micrograms; in depolarizing conditions (10 min perfusion with a solution containing 70 mM K+ but no Ca2+), secretion was somewhat less (4-4.5 micrograms). (+)Isradipine, a 1,4-dihydropyridine Ca2+ channel blocker, did not affect the secretory response under polarizing conditions at 10(-8) M; at 10(-6) M, the secretory response was halved. When present under depolarizing conditions (70 mM K+ in 0 Ca2+), (+)isradipine (10(-8) M) blocked catecholamine release by 90%. In contrast, the inorganic Ca2+ channel blocker, Co2+, inhibited secretion equally well under polarizing or depolarizing conditions. Since 45Ca2+ uptake into adrenal medullary chromaffin cells was also inhibited by (+)isradipine (10(-8) M) in a voltage-dependent manner, it seems likely that blocking effects of the drug on catecholamine release are associated with inhibition of Ca2+ entry into cells through L-type Ca2+ channels. The association of (+)isradipine to its receptor is very rapid under polarizing conditions; dissociation is very slow in depolarized cells and very rapid upon polarization of such cells. Since chromaffin cells are being depolarized during stressful situations to secrete catecholamines into the circulation, (+)isradipine is likely to bind better to dihydropyridine receptors in this state; in this manner, the ensuing blockade of adrenal secretion could serve as a protective mechanism of cardiovascular tissues against massive increases in circulating catecholamines. If this suggestion is correct this mechanism could have additional therapeutic value in the treatment of hypertensive patients with (+)isradipine.

Voltage-dependent inactivation of catecholamine secretion evoked by brief calcium pulses in the cat adrenal medulla

1. Inactivation by voltage changes of 45Ca2+ uptake into and catecholamine release from cat adrenal glands perfused at a high rate (4 ml/min) at 37 degrees C with oxygenated Krebs-Tris solution has been studied. Experimental conditions were selected so that adrenal medullary chromaffin cells were depolarized for different time periods and with various K+ concentrations in the absence of Ca2+, prior to the application of 0.5 mM-Ca2+ for 10 s in the presence of 118 mM-K+ to test the rate of secretion (the 'Ca2+ pulse'). 2. Application of the Ca2+ pulse after perfusion with 5.9 mM-K+ led to a 100-fold increase of the basal rate of secretion. However, if the Ca2+ pulse was preceded by a 10 min period of perfusion with 118 mM-K+, the secretory response was decreased by over 80%. 3. Inactivation of secretion starts 10-30 s after high-K+ perfusion and is completed 2-5 min thereafter. Inactivation is readily reversed by perfusing the glands with normal K(+)-containing solution; the recovery phenomenon is also gradual and time-dependent, starting 30 s after repolarization and ending 300 s thereafter. 4. The rate of inactivation is much slower at 35 than at 118 mM-K+, suggesting that the process is strongly dependent on voltage. 5. Like catecholamine release, Ca2+ uptake into adrenal medullary chromaffin cells is inactivated in a voltage-dependent manner. This, together with the fact that Cd2+ blocked secretion completely and inactivation was seen equally using Ca2+ or Ba2+ as secretagogues, suggests that inactivation of a certain class of voltage-dependent Ca2+ channels is responsible for the blockade of secretion. Such channels must be slowly inactivated by voltage and highly sensitive to dihydropyridines, since (+)PN200-110 (an L-type Ca2+ channel blocker) enhanced the rate of inactivation and (+/-)Bay K 8644 (an L-type Ca2+ channel activator) prevented it, indicating that they might belong to L-subtype Ca2+ channels. 6. The effects of (+/-)Bay K 8644 (100 nM) were seen on both the voltage and time dependence of inactivation. At a moderate depolarization (35 mM-K+), the drug prevented inactivation and caused potentiation of secretion which developed gradually; at strong depolarizations (118 mM-K+), Bay K 8644 prevented the time-dependent development of inactivation. (+)PN200-110 (30 nM) did not suddenly decrease catecholamine release at the earlier times of depolarization; what the drug did was to accelerate the normal rate of inactivation induced by depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)