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

Real Time Recording of Perifused Chromaffin Cells

Amperometry is an electrochemical method based on the oxidation or reduction of molecules. Many secretion products, including catecholamines, contain in their molecule chemical groups with the ability to yield (oxidize) or capture (reduce) electrons upon its exposure to an electrical field. In order to measure the secretion of catecholamines, they are oxidized at +650 mV with a carbon electrode, releasing every molecule of catecholamine that is oxidized two electrons (e^-) that are recorded as an electrical current. Amperometry is an easy-to-use and noninvasive technique for cells (unlike patch-clamp techniques for measuring membrane capacitance) and has been widely used to monitor online catecholamine release from perifused bovine chromaffin cell populations.

Effects of gentamicin and pH on [Ca2+]i in apical and basal outer hair cells from guinea pigs

Aminoglycosides are widely used antibiotics and frequently produce acute ototoxicity. In this study we attempted to comparatively investigate the effects of gentamicin on Ca2+ influx of apical and basal outer hair cells (OHCs) isolated from guinea-pig cochlea. Since the solution of gentamicin sulfate salt is acidic (pH 3.1-3.3), we also explored the effect of external acidification on Ca2+ influx. By means of fura-2 microspectrofluorimetry, we measured the intracellular calcium concentration ([Ca2+]i) of OHCs bathed in Hanks' balanced salt solution (pH 7.40) during either a resting state or high K+-induced depolarization. Our results show that at the resting state, the baseline [Ca2+]i in apical OHCs (94+/-2.0 nM) was slightly lower than that in basal OHCs (101.1+/-2.4 nM). By contrast, the increase in [Ca2+]i evoked by high K+ depolarization in apical OHCs was about two-fold greater than that in basal OHCs. Nifedipine (30 microM) abolished the increased [Ca2+]i in both types of OHCs, suggesting that Ca2+ influx was mainly through L-type Ca2+ channels of OHCs. While gentamicin and extracellular acidification (pH 7.14) can separately attenuate this increase in [Ca2+]i in both types of OHCs, their suppressive effects are additive in basal OHCs, but not in apical OHCs. The implications of these findings are that: (1) apical and basal OHCs behave differently in response to depolarization-increased [Ca2+]i, and (2) basal OHCs are more vulnerable to the impairment of Ca2+ entry during depolarization by a combination of gentamicin and extracellular acidification, which is correlated with the clinical observation that ototoxicity of aminoglycosides at the basal coil of OHCs is more severe than that at the apical coils. Moreover, the possibility that extracellular acidification may enhance the acute ototoxic effects of aminoglycosides should be considered especially in topical applications.

Voltage inactivation of Ca2+ entry and secretion associated with N- and P/Q-type but not L-type Ca2+ channels of bovine chromaffin cells

1. In this study we pose the question of why the bovine adrenal medullary chromaffin cell needs various subtypes (L, N, P, Q) of the neuronal high-voltage activated Ca2+ channels to control a given physiological function, i.e. the exocytotic release of catecholamines. One plausible hypothesis is that Ca2+ channel subtypes undergo different patterns of inactivation during cell depolarization. 2. The net Ca2+ uptake (measured using 45Ca2+) into hyperpolarized cells (bathed in a nominally Ca2+-free solution containing 1.2 mM K+) after application of a Ca2+ pulse (5 s exposure to 100 mM K+ and 2 mM Ca2+), amounted to 0.65 +/- 0.02 fmol cell-1; in depolarized cells (bathed in nominally Ca2+-free solution containing 100 mM K+) the net Ca2+ uptake was 0.16 +/- 0.01 fmol cell-1. 3. This was paralleled by a dramatic reduction of the increase in the cytosolic Ca2+ concentration, [Ca2+]i, caused by Ca2+ pulses applied to fura-2-loaded single cells, from 1181 +/- 104 nM in hyperpolarized cells to 115 +/- 9 nM in depolarized cells. 4. A similar decrease was observed when studying catecholamine release. Secretion was decreased when K+ concentration was increased from 1.2 to 100 mM; the Ca2+ pulse caused, when comparing the extreme conditions, the secretion of 807 +/- 35 nA of catecholamines in hyperpolarized cells and 220 +/- 19 nA in depolarized cells. 5. The inactivation by depolarization of Ca2+ entry and secretion occluded the blocking effects of combined omega-conotoxin GVIA (1 microM) and omega-agatoxin IVA (2 microM), thus suggesting that depolarization caused a selective inactivation of the N- and P/Q-type Ca2+ channels. 6. This was strengthened by two additional findings: (i) nifedipine (3 microM), an L-type Ca2+ channel blocker, suppressed the fraction of Ca2+ entry (24 %) and secretion (27 %) left unblocked by depolarization; (ii) FPL64176 (3 microM), an L-type Ca2+ channel 'activator', dramatically enhanced the entry of Ca2+ and the secretory response in depolarized cells. 7. In voltage-clamped cells, switching the holding potential from -80 to -40 mV promoted the loss of 80 % of the whole-cell inward Ca2+ channel current carried by 10 mM Ba2+ (IBa). The residual current was blocked by 80 % upon addition of 3 microM nifedipine and dramatically enhanced by 3 microM FPL64176. 8. Thus, it seems that the N- and P/Q-subtypes of calcium channels are more prone to inactivation at depolarizing voltages than the L-subtype. We propose that this different inactivation might occur physiologically during different patterns of action potential firing, triggered by endogenously released acetylcholine under various stressful conditions.

Effects of tyramine and calcium on the kinetics of secretion in intact and electroporated chromaffin cells superfused at high speed

Fast superfusion of electroporated bovine adrenal chromaffin cells with a K+ glutamate-based solution containing 50 nM free Ca2+ and 2 mM adenosine 5'-triphosphate, dipotassium salt (K2ATP), produced a steady-state low catecholamine secretion, measured on-line with an electrochemical detector (about 20 nA). Rapid switching to electroporation solutions containing increasing Ca2+ concentrations ([Ca2+]) produced a rapid increase in the rate and peak secretion, followed by a decline. At intermediate [Ca2+] (3-100 microM), a fast peak and a slow secretory plateau were distinguished. The fast secretory peak identifies a readily releasable catecholamine pool consisting of about 200-400 vesicles per cell. Pretreatment of cells with tyramine (10 microM for 4 min before electroporation) supressed the initial fast secretory peak, leaving intact the slower phase of secretion. With [Ca2+] in the range of 0.1-3 microM, the activation rate of secretion increased from 2.3 to 35.3 nA.s-1, reached a plateau between 3-30 microM and rose again from 100 to 1000 microM [Ca2+] to a maximum of 91.9 nA.s-1. In contrast, total secretion first increased (0.1-1 microM Ca2+), then plateaud (1-100 microM Ca2+) and subsequently decreased (100-1000 microM Ca2+). At 30 and 1000 microM extracellular [Ca2+] or [Ca2+]o, the activation rates of secretion from intact cells depolarised with 70 mM K+ were close to those obtained in electroporated cells. However, secretion peaks were much lower in intact (93 nA at 30 microM Ca2+) than in electroporated cells (385 nA). On the other hand, inactivation of secretion was much faster in intact than in electroporated cells; as a consequence, total secretion in a 5-min period was considerably smaller in intact (10.6 microA.s at 1000 microM Ca2+) than in electroporated cells (42.4 microA.s at 1 microM Ca2+). Separation of the time-courses of changes in intracellular [Ca2+] or [Ca2+]i and secretion in intact chromaffin cells depolarised with 70 mM K+ was demonstrated at different [Ca2+]o. The increase in the rate of catecholamine release was substantially higher than the increase of the average [Ca2+]i. In contrast, the decline of secretion was faster than the decline of the peak [Ca2+]i. The results are compatible with the idea that the peak and the amount of catecholamine released from depolarised intact cells is determined essentially by plasmalemmal factors, rather than by vesicle supply from reserve pools. These plasmalemmal factors limit the supply of Ca2+ by the rates of opening and closing of voltage-dependent Ca2+ channels of the L- and Q-subtypes, which control the local [Ca2+]i near to exocytotic sites.

Veratridine-induced oscillations of cytosolic calcium and membrane potential in bovine chromaffin cells

1. Veratridine (VTD) induced large oscillations of the cytosolic Ca2+ concentration ([Ca2+]i) and the membrane potential (Vm) in otherwise silent bovine chromaffin cells loaded with fura-2. 2. Depletion of the intracellular Ca2+ stores by thapsigargin or ryanodine did not affect these oscillations. Caffeine had a complex effect, decreasing them in cells with high activity but increasing them in cells with low activity. 3. The [Ca2+]i oscillations required extracellular Ca2+ and Na+ and were blocked by Ni2+ or tetrodotoxin. They were antagonized by high external concentrations of Mg2+ and/or Ca2+. 4. The oscillations of Vm had three phases: (i) slow depolarization (20 mV in 10-40 s); (ii) further fast depolarization (30 mV in 1 s); and (iii) rapid (5 s) repolarization. [Ca2+]i decreased during (i), increased quickly during (ii) with a 1 s delay with regard to the peak depolarization, and decreased during (iii). 5. Slight depolarizations increased the frequency of the oscillations whereas large depolarizations decreased it. 6. The Ca(2+)-dependent K+ channel blocker apamin increased the duration and decreased the frequency of the oscillations. 7. We propose the following mechanism for the oscillations: (i) the membrane depolarizes slowly by a decrease of potassium conductance (gK), perhaps due to a gradual decrease of [Ca2+]i; (ii) the threshold for activation of Na+ channels (decreased by VTD) is reached, producing further depolarization and recruiting Ca2+ channels, and inactivation of both Ca2+ and VTD-poisoned Na+ channels is slow; and (iii) gK increases, aided by activation of Ca(2+)-dependent K+ channels by the increased [Ca2+]i, and the membrane repolarizes. The contribution of the Na+ channels seems essential for the generation of the oscillations. 8. Bovine chromaffin cells have the machinery required for [Ca2+]i oscillations even though the more physiological stimulus tested here (high K+, field electrical stimulation, nicotinic or muscarinic agonists) produced mainly non-oscillatory responses.

Two components in the adrenal nicotinic secretory response revealed by cobalt ramps

Prolonged stimulation with nicotine (50 microM) enhanced the secretion of catecholamines from perfused cat adrenal glands. The profile of secretion consisted of a quick activation phase to a peak of 7.68 micrograms/min followed by a second inactivation phase which exhibited a t1/2 of 3.75 min. Sustained stimulation with a solution enriched in K+ (59 mM) also evoked a transient secretory response, with a peak release of 8.62 micrograms/2 min and a t1/2 for inactivation of 4.8 min. Co2+ (10 mM) blocked the nicotinic response by 58% and the K(+)-evoked secretory response by over 96%. In the presence of Co2+ (5 mM), continuous perfusion with nicotine produced a transient but large initial secretory response; the gradual decrease of the extracellular Co2+ concentration, [Co2+]0, as a continuous ramp allowed the development of a second component of secretion which inactivated later on. When the glands were continuously stimulated with 59 mM K+ in the presence of Co2+, the first component of secretion was missing; the second component appeared as [Co2+]0 decreases as a ramp. In similar experiments performed in low-Na+ solution (10 mM Na+), only the first secretion component evoked by nicotine was observed. This finding suggests that the second component of secretion depends on Na+ entry through the nicotinic receptor, on the ensuing cell depolarization and on Ca2+ entry through voltage-dependent Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Permeation and inactivation by calcium and manganese of bovine adrenal chromaffin cell calcium channels

Stimulation of fura-2-loaded bovine chromaffin cells with the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP; 10 microM) or depolarization with high [K+] (50 mM) accelerated the entry of both Ca2+ and Mn2+, used here as a Ca2+ surrogate for Ca2+ channels. Removal of extracellular Na+ prevented the effects of DMPP but did not modify the effects of K+, indicating that Na+ is necessary for coupling of Ca2+ entry to the nicotinic receptor activation and that the ionophore associated with it is functionally impermeable to divalent cations. DMPP- as well as K(+)-evoked Ca2+ and Mn2+ influx were blocked completely by Ni2+ but only partially by dihydropyridines, suggesting that, in addition to L-type Ca2+ channels, other Ca2+ entry pathways may be present. Inactivation of Ca2+ channels, followed by comparing the rates of Mn2+ uptake at different time periods after the addition of DMPP or high K+, did not happen in the absence of extracellular Ca2+. When 1 mM Ca2+ was present, a delayed inhibition (half time, 10-20 s) was observed, suggesting that it is not due to the entry of Ca2+ itself but to the increase of the cytoplasmic Ca2+ concentration ([Ca2+]i) that takes a few seconds to develop. The influx of Ca2+, estimated from the increase of [Ca2+]i, was also impaired in a time-dependent fashion by previous entry of Mn2+. Inactivation of Ca2+ entry was achieved at estimated mean intracellular Mn2+ concentrations as low as 10(-9) M.

Inhibition of voltage-dependent Ca2+ channels via alpha 2-adrenergic and opioid receptors in cultured bovine adrenal chromaffin cells

Adrenal chromaffin cells secrete catecholamines and opioids. The effects of these agents on whole-cell Ca2+ channel currents were studied, using bovine adrenal chromaffin cells kept in short term culture. Ca2+ channel currents recorded during voltage-clamp pulses from a holding potential of -80 mV to 0 mV were reversibly reduced by 10 microM epinephrine (in the presence of 1 microM propranolol) or 5 microM of the synthetic opioid, d-Ala2-d-Leu5-enkephalin (DADLE) by approximately 35% and 25%, respectively. The inhibitory action of epinephrine was mimicked by clonidine, reduced by yohimbine but not affected by prazosin. The DADLE-induced reduction of the Ca2+ channel current was antagonized by naloxone. The dihydropyridine (+)PN 200-110 (5 microM) reduced the Ca2+ channel current by approximately 40%; the Ca2+ channel current inhibited by (+)PN 200-110 was not further reduced by epinephrine. Intracellular infusion of guanosine-5'-O-(2-thiodiphosphate) and pretreatment of cells with pertussis toxin abolished the inhibitory effect of both epinephrine and DADLE. In membranes of adrenal chromaffin cells, four pertussis-toxin-sensitive G-proteins were identified, including Gi1, Gi2, Go1 and another Go subtype, possibly Go2. The data show that activation of alpha 2-adrenergic and opioid receptors causes an inhibition of dihydropyridine-sensitive Ca2+ channels in adrenal chromaffin cells. These inhibitory modulations are mediated by pertussis-toxin-sensitive G-proteins and may represent a mechanism for a negative feedback signal by agents released from the adrenal medulla.

Tumor necrosis factor modulates the inactivation of catecholamine secretion in cultured sympathetic neurons

Cytokines exert multiple effects on cellular functions. We studied the effects of cytokines on the calcium-dependent release of catecholamines in cultured neurons from neonatal rat superior cervical ganglia. Incubation of sympathetic neurons with recombinant human interleukin-1 beta (0.14-0.7 nM) or recombinant human tumor necrosis factor-alpha (1 nM) for 24-48 h had no effect on the baseline spontaneous release and the initial K(+)-evoked [3H]norepinephrine release, compared with untreated cells. A repeat K(+)-induced depolarization after 6 min resulted in a decrease of [3H]norepinephrine secretion to 69 +/- 5.8% (n = 11) of the initial secretion in recombinant human tumor necrosis factor-treated cells, but not in control cells. The secretory response was restored when the interval between the two K+ challenges was increased to 10 min. We conclude that the diminished secretory response to a repeat stimulus in recombinant human tumor necrosis factor-treated superior cervical ganglia neurons is due to a prolonged recovery from inactivation of secretion in these cells.

(+)-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.