An inhibitory effect of extracellular Ca2+ on Ca2+-dependent exocytosis

Pharmacological implications of the Ca(2+)/cAMP signaling interaction: from risk for antihypertensive therapy to potential beneficial for neurological and psychiatric disorders

In this review, we discussed pharmacological implications of the Ca²⁺/cAMP signaling interaction in the antihypertensive and neurological/psychiatric disorders therapies. Since 1975, several clinical studies have reported that acute and chronic administration of L-type voltage-activated Ca²⁺ channels (VACCs) blockers, such as nifedipine, produces reduction in peripheral vascular resistance and arterial pressure associated with an increase in plasma noradrenaline levels and heart rate, typical of sympathetic hyperactivity. Despite this sympathetic hyperactivity has been initially attributed to adjust reflex of arterial pressure, the cellular and molecular mechanisms involved in this apparent sympathomimetic effect of the L-type VACCs blockers remained unclear for decades. In addition, experimental studies using isolated tissues richly innervated by sympathetic nerves (to exclude the influence of adjusting reflex) showed that neurogenic responses were completely inhibited by L-type VACCs blockers in concentrations above 1 μmol/L, but paradoxically potentiated in concentrations below 1 μmol/L. During almost four decades, these enigmatic phenomena remained unclear. In 2013, we discovered that this paradoxical increase in sympathetic activity produced by L-type VACCs blocker is due to interaction of the Ca²⁺/cAMP signaling pathways. Then, the pharmacological manipulation of the Ca²⁺/cAMP interaction produced by combination of the L-type VACCs blockers used in the antihypertensive therapy, and cAMP accumulating compounds used in the antidepressive therapy, could represent a potential cardiovascular risk for hypertensive patients due to increase in sympathetic hyperactivity. In contrast, this pharmacological manipulation could be a new therapeutic strategy for increasing neurotransmission in psychiatric disorders, and producing neuroprotection in the neurodegenerative diseases.

Detection of insulin granule exocytosis by an electrophysiology method with high temporal resolution reveals enlarged insulin granule pool in BIG3-knockout mice

We recently identified BIG3 as a negative regulator of insulin granule biogenesis and reported increased insulin secretion in BIG3-knockout (BKO) mice. To pinpoint the site of action for BIG3, we investigated whether BIG3 regulates quantal insulin granule exocytosis. We established an assay to detect insulin granule exocytosis by recording ATP-elicited currents at high temporal resolution by patch clamp. Similarly to insulin, ATP release was increased in BKO β-cells. Although the frequency of insulin granule exocytosis was increased in BKO β-cells, quantal size or release kinetics remained unchanged. Electron microscopy studies showed that the number of insulin granules was increased by >60% in BKO β-cells. However, the number of morphologically docked granules was unaltered. The number of insulin granules having significant distances away from plasma membrane was greatly increased in BKO β-cells. Thus, BIG3 negatively regulates insulin granule exocytosis by restricting insulin granule biogenesis without the release kinetics of individual granules at the final exocytotic steps being affected. Depletion of BIG3 leads to an enlarged releasable pool of insulin granules, which accounts for increased release frequency and consequently increased insulin secretion.

Extracellular Ca²⁺ per se inhibits quantal size of catecholamine release in adrenal slice chromaffin cells

Classic calcium hypothesis states that depolarization-induced increase in intracellular Ca(2+) concentration ([Ca(2+)]i) triggers vesicle exocytosis by increasing vesicle release probability in neurons and neuroendocrine cells. The extracellular Ca(2+), in this calcium hypothesis, serves as a reservoir of Ca(2+) source. Recently we find that extracellular Ca(2+)per se inhibits the [Ca(2+)]i dependent vesicle exocytosis, but it remains unclear whether quantal size is regulated by extracellular, or intracellular Ca(2+) or both. In this work we showed that, in physiological condition, extracellular Ca(2+) per se specifically inhibited the quantal size of single vesicle release in rat adrenal slice chromaffin cells. The extracellular Ca(2+) in physiological concentration (2.5 mM) directly regulated fusion pore kinetics of spontaneous quantal release of catecholamine. In addition, removal of extracellular Ca(2+) directly triggered vesicle exocytosis without eliciting intracellular Ca(2+). We propose that intracellular Ca(2+) and extracellular Ca(2+)per se cooperately regulate single vesicle exocytosis. The vesicle release probability was jointly modulated by both intracellular and extracellular Ca(2+), while the vesicle quantal size was mainly determined by extracellular Ca(2+) in chromaffin cells physiologically.

Roles of the calcium sensing receptor in the central nervous system

The calcium sensing receptor (CaSR) is expressed by subpopulations of neuronal and glial cells throughout the brain and is activated by extracellular calcium [Formula: see text] . During development, the CaSR regulates neuronal cell growth and migration as well as oligodendroglial maturation and function. Emerging evidence suggests that in nerve terminals, CaSR is implicated in synaptic plasticity and neurotransmission. In this review, we analyze the roles attributed to CaSR in regulating diverse brain functions, including central regulation of body fluid composition and blood pressure. We also discuss the potential relevance of Ca(2+)-sensing in brain by other family C G protein-coupled receptors. Finally, evidence that the CaSR contributes to the pathogenesis of various brain disorders raises the possibility that pharmacological modulators of the CaSR may have therapeutic benefit.