Alpha- and Beta-adrenergic Stimulation of Parenchymal Cell Ca2+ Influx. Influence of Extracellular PH

Stimulation of parenchymal cell alpha- and beta-adrenergic receptors, with either epinephrine or the specific alpha-agonist phenylephrine or the beta-agonist isoproterenolol, promoted a net Ca2+ influx when the pH of the external medium was greater than pH 7.7. The effect was both time- and dose-dependent. At the elevated pH of 8.1, alpha-adrenergic receptors were still able to mobilize intracellular Ca2+, since when the external Ca2+ concentration was reduced to 0.05 mM, epinephrine promoted net Ca2+ efflux. The data suggest the existence of a Ca2+/H+ antiport or a Ca2+/OH- symport in the plasma membrane which can be stimulated by either alpha- or beta-adrenergic receptors. It is proposed that the Ca2+ influx mediated by alpha-adrenergic receptors helps maintain the elevated cytosolic [Ca2+]i for longer time periods after the initial mobilization from intracellular Ca2+ stores.