EVIDENCE THAT ENDOGENOUS SOMATOSTATIN (SRIF) EXERTS AN INHIBITORY CONTROL ON THE FUNCTION AND GROWTH OF RAT ADRENAL ZONA GLOMERULOSA - THE POSSIBLE INVOLVEMENT OF ZONA MEDULLARIS AS A SOURCE OF ENDOGENOUS SRIF

The effect of SRIF and its antagonist (7-aminoheptanonyl-Phe-D-Trp-Lys -Thr\Bzl\)(SRIF-A) were studied in sham-operated and bilaterally adren alectomized rats bearing ACTH- and angiotensin II (ANG-II)-responsive adrenocortical autotransplants. SRIF-A (10(-5) M) completely annulled SRIF (10(-6) M)-induced inhibition of ANG-II (10(-8) M)-evoked rise in aldosterone (ALDO) secretion by both dispersed zona glomerulosa (ZG) cells and autotransplant slices. A 7-day intraperitoneal infusion with SRIF (0.3 nmol.kg(-1).min(-1)) significantly lowered plasma ALDO conc entration (PAC) in both groups of animals, without affecting plasma re nin activity and the plasma levels of ACTH and corticosterone. This tr eatment caused a marked atrophy of adrenal ZG and its parenchymal cell s (without inducing any significant change in the zona fasciculata mor phology), as well as of ZG-like cells of autotransplants. Isolated ZG cells and autotransplant slices from SRIF-infused rats evidenced a not able decrease in both their basal and maximally ACTH- or ANG-II-stimul ated ALDO production. The simultaneous infusion of rats with SRIF-A (3 nmol.kg(-1).min(-1)) completely reversed all these effects of SRIF. T he prolonged infusion with SRIF-A alone caused, in sham-operated rats, a marked increase in PAC and a significant hypertrophy of ZG and ZG c ells; basal and maximally-stimulated ALDO secretion of dispersed ZG ce lls was also notably raised. Conversely, SRIF-A infusion did not evoke any appreciable effect in autotransplanted rats. These findings sugge st that endogenous SRIF is specifically involved in the negative contr ol of the secretion and growth of the rat adrenal ZG. Since regenerate d adrenocortical autotransplants, which are responsive to SRIF but not to SRIF-A infusion, are completely deprived of chromaffin cells, the hypothesis is advanced that adrenal zona medullaris may be the source of endogenous SRIF regulating ZG function.