Increased steroidogenesis by rat zona glomerulosa cells with increased cell concentration in vitro: evidence for a novel aldosterone-stimulating factor and implications regarding aldosterone biosynthesis.

A fourfold increase in the concentration of rat zona glomerulosa cells incubated in Krebs-Ringer bicarbonate buffer (KRBGA) increased aldosterone production per cell twofold. This cell concentration associated (CCA) increase in aldosterone production showed a dose-response relationship with the number of cells per incubation and was further increased in modified Medium 199, compared with KRBGA. The combination of a fourfold increase in cell concentration with each of maximally steroidogenic concentrations of K+, angiotensin II, ACTH, 5-hydroxytryptamine or cyclic AMP produced an increase in aldosterone production greater than that predicted from the algebraic sum of the steroidogenic effects of the factors acting separately. The CCA increase in basal aldosterone production maximal during the first 30 min of incubation, could not be accounted for by accumulation of corticosterone in the incubation medium and was largely or completely abolished by the addition of pregnenolone, progesterone, deoxycorticosterone or corticosterone to the incubation medium. Together, these data support the proposal that the CCA effect was due to a novel aldosterone-stimulating factor which increased aldosterone production by stimulating pregnenolone formation. The aldosterone and corticosterone responses to exogenous pregnenolone, progesterone, deoxycorticosterone and corticosterone demonstrated that aldosterone and corticosterone production by zona glomerulosa cells in vitro may represent separate biosynthetic pathways. Furthermore, these studies provided evidence for a mechanism by which increased levels of exogenous pregnenolone, progesterone and deoxycorticosterone were able to increase their per cent conversion to aldosterone, but not to corticosterone.