Effect of glucagon on calcium binding by rat liver plasma membranes

Calcium binding by parathyroid cell plasma membranes

Parathyroid hormone (PTH) secretion from parathyroid glands is controlled mainly by extracellular calcium both in vivo and in vitro. In this study, the Ca2+ binding properties of bovine parathyroid cells have been investigated on a highly purified plasma membrane preparation with flow dialysis techniques. Scatchard plot analysis of the data shows the existence of at least two different binding sites: the high affinity have an apparent Kd1 of 6.6 X 10(-5) M and a capacity (n1) of 20.1 nmol/mg protein. The low affinity sites have an apparent Kd2 of 2.6 X 10(-4) M and a capacity (n2) of 37.7 nmol/mg protein. Furthermore, at higher total Ca2+ concentrations, additional bindings with an apparent Kd3 in the millimolar range and a capacity (n3) of 118 nmol/mg protein are detectable. Neither the apparent Kd's nor n's of these calcium binding sites was affected by isotonic substitution of the medium with NaCl or KCl, while the number of binding sites increased in Choline Cl. On the other hand, the presence of 5-10 mM LiCl in isotonic Na-K medium caused a marked decrease in calcium binding from these sites. LiCl is the only monovalent cation that both in vitro and in vivo is able to enhance PTH release regardless of high or low extracellular Ca2+ concentrations. Therefore the observed displacement of calcium from calcium binding sites seems to be rather specific and could correlate with the observed enhanced PTH secretion in parathyroid cells. It is proposed that the first event in the regulation of PTH secretion by extracellular Ca2+ is the binding of Ca2+ to the low affinity, high capacity sites present on the surface of parathyroid cells and that calcium occupancy of those sites is the necessary event to initiate intracellular signals leading to inhibition of PTH secretion.

Perturbations of liver plasma membranes induced by Ca2+ are detected using a fatty acid spin label and adenylate cyclase as membrane probes

Ca2+ decreased the lipid fluidity of rat liver plasma membranes labeled with 5-nitroxide stearate, I(12,3), as indicated by the order parameter (S). These effects form a reversible, saturable process with an association constant of 1 x 10(3) M-1. Arrhenius-type plots of S indicated that the lipid phase separation, present in the external leaflet of native membranes between 28 and 19 degrees C, is perturbed by mM Ca2+ such that the high temperature onset is elevated to 32-34 degrees C. Fluoride-stimulated adenylate cyclase was similarly inhibited by Ca2+ (ID50 = 1 mM) for the enzyme in membrane-bound or solubilized states. The glucagon-stimulated activity was more sensitive to Ca2+ inhibition with an ID50 of 0.2 mM. These inhibitory effects are due neither to perturbations of glucagon binding to its receptor nor to fluidity changes, but are instead attributed to direct Ca2+-enzyme interactions. Such binding desensitizes the enzyme to fluidity alterations induced by temperature elevation or benzyl alcohol addition. With Ca2+, Arrhenius plots of glucagon-stimulated activity indicated breaks at 32 and 16 degrees C, whereas those of fluoride-stimulated activity showed one break at 17 degrees C. Without Ca2+, Arrhenius plots exhibited one break at 28 degrees C for glucagon-stimulated activity, whereas fluoride-stimulated plots were linear. We propose that Ca2+ achieves these effects through asymmetric perturbations of the membrane lipid structure.