Histamine and a guanine nucleotide increase calcium permeability in pig aortic microsomal fractions

ATP-dependent Ca2+ accumulation was measured in pig aortic microsomal fractions containing plasmalemma and endoplasmic reticulum. In vesicles sonicated with histamine, to allow access to internally located receptor sites, guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG), added to activate externally located guanine-nucleotide-transducing proteins, caused a concentration-dependent decrease in steady-state Ca2+ accumulation that was reversed by guanosine 5'-[beta-thio]diphosphate. In the presence of p[NH]ppG, sonication with histamine produced a concentration-dependent inhibition of Ca2+ accumulation that could be antagonized by the H1 antagonist mepyramine, but not by the H2 antagonist cimetidine. The inhibition of steady-state Ca2+ accumulation could have resulted from an inhibition of ATP-dependent Ca2+ uptake or a stimulation of Ca2+ release. We observed, however, that p[NH]ppG plus histamine stimulated, rather than inhibited, Ca2(+)-ATPase activity. We concluded that p[NH]ppG and histamine acted together to increase Ca2+ permeability. In support of this, p[NH]ppG accelerated efflux of Ca2+ from passively loaded vesicles sonicated with, but not without, histamine. The effect of p[NH]ppG was unlikely to be due to Ins(1,4,5)P3 (and hence release from endoplasmic-reticulum vesicles), since addition of Ins(1,4,5)P3 to vesicles sonicated with histamine did not alter steady-state Ca2+ accumulation. Our results therefore suggest that histamine and p[NH]ppG increased the permeability of the plasmalemma vesicles and may thus model the process of receptor-mediated Ca2+ entry into intact cells

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiacryanodine receptor-Ca2+ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca2+ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology

Physical coupling between ryanodine receptor-calcium release channels

Ryanodine receptor–calcium release channels play a pivotal role in the calcium signaling that mediates muscle excitation–contraction coupling. Their membrane organization into regular patterns, functional gating studies and theoretical analysis of receptor clustering have led to models that invoke allosteric interaction between individual channel oligomers as a critical mechanism for control of calcium release. Here we show that in reconstituted “checkerboard-like� lattices that mimic in situ membrane channel arrays, each oligomer is interlocked physically with four adjacent oligomers via a specific domain–domain interaction. Direct physical coupling between ryanodine receptors provides structural evidence for an inter-oligomer allosteric mechanism in channel regulation. Therefore, in addition to established cytosolic and luminal regulation of function, these observations indicate that channel–channel communication through physical coupling provides a novel mode of regulation of intracellular calcium release channels

Vasoconstrictor agonists activate G-protein-dependent receptor-operated calcium channels in pig aortic microsomes

Receptor-operated Ca2+ channels were characterized by their ability to decrease steady-state ATP-dependent Ca2+ accumulation into pig aortic microsomes. The vasoconstrictor agents noradrenaline, angiotensin II and adenosine 5'-[alpha beta-methylene]triphosphate (pp[CH2]pA) all decreased Ca2+ accumulation only when sonicated into vesicles (to allow access to receptor sites) and in the presence of guanosine 5'-[beta gamma-imido]triphosphate to activate transducing G-proteins. The effect of noradrenaline was inhibited by the alpha 2 antagonist yohimbine, but not by the alpha 1 antagonist prazosin. The effect of none of the agonists was reversed by diltiazem. SK&F 96365 (an inhibitor of receptor-mediated Ca2+ influx into intact cells) reversed the effect of noradrenaline, but not that of pp[CH2]pA, which suggests that at least two receptor-operated channels may be present in this preparation