POTASSIUM NEGATIVELY REGULATES ANGIOTENSIN-II TYPE-1 RECEPTOR EXPRESSION IN HUMAN ADRENOCORTICAL H295R CELLS

We have previously shown that the human adrenocortical H295R cell line expresses the type 1 angiotensin II receptor (AT(1)-R) and that expression of this receptor is downregulated at the level of mRNA by forskolin or dibutyryl cAMP as well as by angiotensin II (Ang II). In this study we examine the effects of K+ on both AT(1)-R mRNA and receptors, as monitored through I-125-Ang II binding in the presence of PD 123319. After treatment with a maximal stimulatory steroidogenic dose of K+ (14 mmol/L), H295R cells showed an increase in cytosolic free Ca2+ from 113 to 212 nmol/L. Unlike the effects of Ang II, this increase could be abolished by pretreatment with the Ca2+ channel antagonist nifedipine (1 mu mol/L). AT(1)-R mRNA levels also fell in response to elevated extracellular K+ in a dose-dependent (K-d 9 mmol/L; maximal fall in message at 12 mmol/L) and time-dependent (maximum 50% at 12 hours) manner. The change in AT(1)-R mRNA level was less rapid than that in response to activation of phosphoinositidase C by Ang II or adenylyl cyclase by forskolin or by dibutyryl-cAMP. Unlike the action of Ang II but similar to the action of forskolin or dibutyryl-cAMP, the action of K+ was sustained. Changes in mRNA level in response to treatment with K+, Ang II, Or dibutyryl-cAMP were also paralleled by changes in I-125-Ang II binding in each case. The mechanism of action of K+ on AT(1)-R mRNA also appears to be mediated through the opening of voltage-sensitive channels on the plasma membrane because the drop in AT(1)-R mRNA was similarly abolished by the Ca2+ channel blocker nifedipine. In conclusion, our findings show that AT(1)-R mRNA levels can be controlled through a Ca2+-dependent signaling pathway, as well as through phosphoinositidase C or adenylyl cyclase signaling pathways, and that these changes in mRNA level underlie a corresponding change in receptor protein at the cell surface.