Almost all actions of A-II are mediated through the AT₁ receptor, including its effects on vasoconstriction and aldosterone production and cell growth (Fig. 79-6).The AT₁ receptor is part of the superfamily of peptide hormone receptors with seven membrane-spanning regions linked to G proteins (Fig. 79-7). AT₁ activates phospholipase C, which in turn hydrolyzes phosphoinositide to inositol triphosphate54 These changes increase the intracel-(IP₃) and diacylglycerol. These changes increase the intracellular calcium levels that in turn activate protein kinases. Also, AT₁ receptors appear to lower levels of intracellular
cAMP, an effect that is most evident in the adrenal gland on the control of aldosterone secretion.54