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The effects of intracellular application of two novel Ca2+ releasing agents have been studied in cultured rat dorsal root ganglion (DRG) neurones by monitoring Ca(2+)-dependent currents as a physiological index of raised free cytosolic Ca2+ ([Ca2+]i). A protein based sperm factor (SF) extracted from mammalian sperm, has been found to trigger Ca2+ oscillations and to sensitize unfertilized mammalian eggs to calcium induced calcium release (CICR). In this study intracellular application of SF activated Ca(2+)-dependent currents in approximately two-thirds of DRG neurones. The SF induced activity was abolished by heat treatment, attenuated by increasing the intracellular Ca2+ buffering capacity of the cells and persisted when extracellular Ca2+ was replaced by Ba2+. In addition, activity could be triggered or potentiated by loading the cells with Ca2+ by activating a series of voltage-gated Ca2+ currents. Ca(2+)-activated inward current activity was also generated by intracellular application of cyclic ADP-ribose (cADPR), a metabolite of NAD+, which causes Ca2+ release in sea urchin eggs. This activity could also be enhanced by loading the cells with Ca2+. The cADPR induced activity, but not the SF induced activity, was abolished by depleting the caffeine sensitive Ca2+ store. Ruthenium red markedly attenuated SF induced activity but had little action on cADPR induced activity or caffeine induced activity. Our results indicate that both SF and cADPR release intracellular Ca2+ pools in DRG neurones and that they appear to act on subtly distinct stores or distinct intracellular Ca2+ release mechanisms, possibly by modulating CICR.
The interaction of bovine platelets with bovine glomerular basement membrane has been studied by aggregometry, transmission and scanning electron microscopy and measurement of [3H] serotonin release. In the absence of added calcium platelets adhere to basement membrane but fail to undergo the release reaction or aggregation. In the presence of 0.2-0.5 mM calcium release of serotonin and complete aggregation of the platelets are observed when sufficient basement membrane is present. Platelets were strongly adhered to the basement membrane surface, the platelet surface in the aggregates closely following the surface of the basement membrane. Platelet morphology in aggregates with basement membrane closely resembled that of platelets from collagen-induced aggregates. Basement membrane differed from collagen in its requirement for calcium for the aggregation and release reactions. In addition purified basement membrane was 1.5-3 fold less active on a weight basis than bovine tendon collagen in promoting aggregation.
Epinephrine rapidly activates phosphorylase in hepatocytes, mainly by a mechanism(s) involving alpha-adrenergic and not beta-adrenergic receptors. The alpha-adrenergic mechanism does not involve accumulation of cAMP or activation of cAMP-dependent protein kinase. It is impaired when hepatocytes are depleted of calcium by EGTA treatment and is rapidly restored by readdition of calcium. Basal phosphorylase is also lowered by calcium deficiency and rapidly increased by calcium but not other divalent cations. The divalent cation ioniphore A23187 increases phosphorylase a levels in hepatocytes in a calcium-dependent manner. Calcium deficiency does not modify the effects of glucagon, cAMP, or beta-adrenergic activation on phosphorylase. Activation of alpha-adrenergic receptors rapidly increases 45Ca fluxes in hepatocytes. Glucagon produces similar effects, but supraphysiological concentrations are required. The hypothesis is advanced that alpha-adrenergic activation of phosphorylase involves alterations in cell calcium such that there is an increase in cytosolic Ca2+ concentration leading to increased phosphorylase kinase activity. Epinephrine induces greater cAMP accumulation in calcium-depleted cells than in normal cells. The effect is mediated by alpha-adrenergic and not beta-adrenergic receptors. Calcium deficiency also cuases cAMP accumulation in hepatocytes incubated with phenylephrine but does not modify the responses of the cells to isoproterenol, glucagon, or cAMP. Low concentrations of calcium rapidly reverse alpha-adrenergic receptor-mediated cAMP accumulation in calcium-depleted cells. The hypothesis is advanced that calcium normally exerts an inhibitory effect on a linkage between alpha-adrenergic receptors and adenylate cyclase in hepatocytes.
Studies were conducted on purified sarcoplasmic reticulum isolated from myotonic goats, an animal model of heritable myotonia. When compared to sarcoplasmic reticulum from normal goats, fragmented sarcoplasmic reticulum from the myotonic goat had (1) increased levels of calcium, (2) increased rates of calcium uptake and efflux, (3) an increased sialic acid content, and (4) an increased content of saturated fatty acids. These differences support the concept of a structural and functional defect as a basis for the abnormal contraction-relaxation characteristics of myotonia.