The Effect of Adrenocorticotropin and Nucleotides on Ca²⁺ Uptake in Adrenal Cortical Microsomal Vesicles

Rat and bovine adrenal cortical microsomal fractions isolated at 27, 000 × g and 105, 000 × g accumulated Ca²⁺ by a nonmitochondrial, ATP-dependent uptake system that was stimulated by ammonium oxalate. ACTH (2 mU/ml) significantly increased Ca²⁺ uptake in bovine adrenal cortical microsomes and in adrenal microsomes from acutely hypophysectomized rats, but only when the hormone was preincubated with intact tissue and not when it was added after homogenization. ACTH did not stimulate Ca²⁺ uptake in adrenal microsomes isolated from nonhypophysectomized, ether-stressed rats, in which basal Ca²⁺ uptake was higher than that observed in microsomes from hypophysectomized animals. The peptides oxytocin, insulin, and TSH did not stimulate Ca²⁺ uptake by adrenal cortical microsomes. ACTH preincubated with intact tissue had no effect on Ca²⁺ uptake in microsomes from liver, kidney, esophagus, or aorta. cAMP, 5′-AMP, and several other nucleotides, nucleosides, and related compounds stimulated adrenal cortical microsomal Ca²⁺ uptake by as much as 540% of control. The stimulatory effects of nucleotides, unlike those of ACTH, were apparent even when the agents were added after homogenization. However, like ACTH, the nucleotides were unable to stimulate Ca²⁺ uptake when they were added to isolated membrane vesicles during Ca²⁺ uptake measurements. It is suggested that the microsomal Ca²⁺ uptake system may respond to physiological stimulants and regulate Ca²⁺ availability in the intact cell.