The Ca²⁺-mobilizing actions of vasopressin and angiotensin differ from those of the α-adrenergic agonist phenylephrine in the perfused rat liver

A Ca²⁺-sensitive electrode was used to study net Ca²⁺-flux changes induced by the administration of phenylephrine, vasopressin and angiotensin to the perfused rat liver. The studies reveal that, although the Ca²⁺ responses induced by vasopressin and angiotensin are similar, they are quite different from the Ca²⁺ fluxes induced by phenylephrine. The administration of phenylephrine is accompanied by a stimulation of a net amount of Ca²⁺ efflux (140 nmol/g of liver). A re-uptake of a similar amount of Ca²⁺ occurs only after the hormone is removed. In contrast, the administration of vasopressin or angiotensin to livers perfused with 1.3 mM-Ca²⁺ induces the release of a relatively small amount of Ca²⁺ (approx. 40 nmol/g of liver) during the first 60 s. This is followed by a much larger amount of Ca²⁺ uptake (70-140 nmol/g of liver) after 1-2.5 min of hormone administration, and a slow efflux or loss of a similar amount of Ca²⁺ over a period of 6-8 min. At lower concentrations of perfusate Ca²⁺ (less than 600 μM) these hormones induce only a net efflux of the ion. These results suggest that at physiological concentrations of extracellular Ca²⁺ the mechanism by which α-adrenergic agonists mobilize cellular Ca²⁺ is different from that involving vasopressin and angiotensin. It seems that the hormones may have quite diverse effects on Ca²⁺ transport across the plasma membrane and perhaps organellar membranes in liver.