Histamine evoked sustained elevations of cytosolic Ca2+ in bovine adrenal chromaffin cells independently of Ca2+ entry

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.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Phospholipase C-mediated signalling is not required for histamine-induced catecholamine secretion from bovine chromaffin cells

A possible role for signalling through phospholipase C in histamine-induced catecholamine secretion from bovine adrenal chromaffin cells has been investigated. Secretion evoked by histamine over 10 min was not prevented by inhibiting inositol-1,4,5-trisphosphate receptors with 2-APB, by blocking ryanodine receptors with a combination of ryanodine and caffeine, or by depleting intracellular Ca(2+) stores by pretreatment with thapsigargin. Inhibition of protein kinase C with Ro31-8220 also failed to reduce secretion. Inhibition of phospholipase C with ET-18-OCH(3) reduced both histamine- and K(+) -induced inositol phosphate responses by 70-80% without reducing their secretory responses. Stimulating phospholipase C with Pasteurella multocida toxin did not evoke secretion or enhance the secretory response to histamine. The secretory response to histamine was little affected by tetrodotoxin or by substituting extracellular Na(+) with N -methyl-d-glucamine(+) or choline(+), or by substituting external Cl(-) with nitrate(-). Blocking various K(+) channels with apamin, charybdotoxin, Ba(2+), tetraethylammonium, 4-aminopyridine, tertiapin or glibenclamide failed to reduce the ability of histamine to evoke secretion. These results indicate that histamine evokes secretion by a mechanism that does not require inositol-1,4,5-trisphosphate-mediated mobilization of stored Ca(2+), diacylglycerol-mediated activation of protein kinase C, or activation of phospholipase C. The results are consistent with histamine acting by depolarizing chromaffin cells through a phospholipase C-independent mechanism.

Histamine promotes excitability in bovine adrenal chromaffin cells by inhibiting an M-current

The current study has investigated the electrophysiological responses evoked by histamine in bovine adrenal chromaffin cells using perforated-patch techniques. Histamine caused a transient hyperpolarization followed by a sustained depolarization of 7.2 +/- 1.4 mV associated with an increase in spontaneous action potential frequency. The hyperpolarization was abolished after depleting intracellular Ca(2+) stores with thapsigargin (100 nM), and was reduced by 40 % with apamin (100 nM). Membrane resistance increased by about 60 % during the histamine-induced depolarization suggesting inhibition of a K(+) channel. An inward current relaxation, typical of an M-current, was observed in response to negative voltage steps from a holding potential of -30 mV. This current reversed at -81.6 +/- 1.8 mV and was abolished by the M-channel inhibitor linopirdine (100 microM). During application of histamine, the amplitude of M-currents recorded at a time corresponding with the sustained depolarization was reduced by 40 %. No inward current rectification was observed in the range -150 to -70 mV, and glibenclamide (10 microM) had no effect on either resting membrane potential or the response to histamine. The results show that an M-current is present in bovine chromaffin cells and that this current is inhibited during sustained application of histamine, resulting in membrane depolarization and increased discharge of action potentials. These results demonstrate for the first time a possible mechanism coupling histamine receptors to activation of voltage-operated Ca(2+) channels in these cells.