Regulation of hormone-sensitive calcium influx by the adenylyl cyclase system in renal epithelial cells

To study signaling pathways regulated by α(s) and α(i1) in renal epithelial cells, we expressed mutant, activated forms of α(s) and α(i1) in a continuous proximal tubule cell line (MCT cells). α(sQ227L) increased cAMP production, and α(i1Q204L) reduced forskolin-sensitive cAMP production. α(i1Q204L) increased and α(sQ227L) decreased bradykinin-induced Ca influx across the cell membrane, but neither mutant affected bradykinin-stimulated intracellular Ca release or basal Ca influx. Bradykinin-stimulated Ca influx was reduced by dibutyryl cAMP, isoproterenol, and forskolin. Expression of a mutant regulatory type I subunit for cAMP-dependent protein kinase with reduced affinity for cAMP and treatment with KT-5720, a specific cAMP- dependent protein kinase inhibitor, enhanced Ca influx to a degree similar to that in cells expressing α(i1Q204L). Bradykinin-stimulated c-fos mRNA expression is partially dependent on extracellular Ca. α(sQ227L) reduced and α(i1Q204L) enhanced bradykinin-stimulated c-fos expression. Consequently, in bradykinin-stimulated cells, the adenylyl cyclase system regulates Ca influx through cAMP-dependent protein kinase, but not intracellular Ca release. Furthermore, the Ca influx mechanism acts as an integrator of two signaling pathways such that Ca-dependent signals are damped by activators of adenylyl cyclase and enhanced by inhibitors of adenylyl cyclase.