Ca2+ calmodulin-dependent protein kinase II activation and regulation of adrenal glomerulosa Ca2+ signaling

We recently reported that elevations in the intracellular Ca2+ concentration ([Ca2+](i)) enhance low-voltage-activated, T-type, Ca2+ channel activity via Ca2+/calmodulin-dependent protein kinase II (CaMKII). Here, we document CaMKII activity in bovine adrenal glomerulosa (AG) cells and assess the importance of CaMKII in depolarization-induced Ca2+ signaling. AG cell extracts exhibited kinase activity toward a CaMKII-selective peptide substrate that was dependent on both Ca2+ [half-maximal concentration for Ca2+ activation (K-0.5) = 1.5 mu M] and calmodulin (K-0.5 = 46 nM) and was sensitive to a calmodulin antagonist and a CaMKII peptide inhibitor. On cell treatment with elevated extracellular potassium (10-60 mM) or angiotensin II, Ca2+-independent CaMKII activity increased to 133-205% of basal activity. Ca2+-independent kinase activity in agonist-stimulated extracts was inhibited by the CaMKII inhibitor peptide, 1-[N,O-bis(1,5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62), a cell-permeable inhibitor of CaMKII, reduced the agonist-induced stimulation of Ca2+-independent CaMKII activity. KN-62 also diminished depolarization-induced increases in [Ca2+](i) without affecting the membrane potential. These observations suggest that CaMKII is activated in situ by aldosterone secretagogues and augments Ca2+ signaling through voltage-gated Ca2+ channels.