Possible involvement of reversible phosphorylation and dephosphorylation of myosin light chain (MLC) by myosin light chain kinase (MLCK) and protein phosphatases (PPases), respectively, in the Ca++-calmodulin-dependent inhibition of renin secretion was investigated with the use of putative MLCK inhibitor ML-7 [1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine] and PPase type1 (PPase-1) and type 2A (PPase-2A) inhibitor calyculin A. ML-7 (1 x 10(-)6 to 3 x 10(-)5 M) increased renin secretion in vitro from rat renal cortical slices under "resting" conditions in a concentration-dependent manner with maximal 2.5-fold stimulation. Furthermore, Ca++-induced inhibition of renin secretion in depolarizing K+-rich Krebs-Ringer bicarbonate not only was prevented completely but also reversed by ML-7 in a concentration-dependent and reversible manner. On the other hand, calyculin A (3 x 10(-)6 M) blocked both effects of ML-7 on stimulation and reversal of renin secretion independently of intracellular Ca++ concentrations. Such antagonistic effects of ML-7 and calyculin A on renin secretion most likely resulted from their respective effects on the level of MLC phosphorylation: ML-7 stimulates renin secretion by decreasing phosphorylation of MLC through its inhibition of MLCK, whereas calyculin A inhibits secretion by increasing phosphorylation of MLC through its inhibition of PPase-1. By inference from these results, MLC may be the target protein involved in regulation of the renin secretory process by Ca++: Ca++-calmodulin phosphorylates MLC via activating MLCK and thereby inhibits renin secretion, whereas dephosphorylation of phosphorylated MLC by PPase-1 reverses the inhibited secretion. We therefore conclude that reversible phosphorylation of MLC may be an important biochemical step determining the rate of renin secretion from the juxtaglomerular cell.