Abstract

In the present study we investigated the cross talk between the Ca²⁺ mobilization pathway and the mitogen-activated protein (MAP) kinase pathway and contraction in the cat iris sphincter smooth muscle. Three Ca²⁺-mobilizing agonists, namely, prostaglandin F_2α (PGF_2α), ionomycin, and thapsigargin, and three specific inhibitors, PD98059, a p42/p44 MAP kinase inhibitor; KN-93, a Ca²⁺-calmodulin-dependent protein kinase II (CaMKII) blocker; and isoproterenol, a cAMP-elevating agent, were used. Changes in tension in response to the agonists were recorded isometrically and MAP kinase phosphorylation and activation were monitored by Western blotting and by in situ myelin basic protein phosphorylation, respectively. We found that 1) stimulation of the sphincter muscle with PGF_2α, ionomycin, or thapsigargin resulted in rapid phosphorylation and activation of p42/p44 MAP kinase and contraction; and 2) treatment of the muscles with PD98059, KN-93, or isoproterenol resulted in inhibition of the Ca²⁺-mobilizing agonist-induced responses. The contractile responses induced by PGF_2α, ionomycin, and thapsigargin were (mg of tension/mg of wet weight tissue) 15.2, 15.4, and 16.2, respectively; the increases in MAP kinase phosphorylation by these agonists were 228, 203, and 190%, respectively; and the increases in MAP kinase activation by the agonists were 212, 191, and 162%, respectively. The stimulatory effects of the agonists on contraction and on MAP kinase phosphorylation and activation were blocked by preincubation of the muscle with PD98059, KN-93, or isoproterenol. These data demonstrate that in the iris sphincter phosphorylation and activation of p42/p44 MAP kinases by PGF_2α, ionomycin, or thapsigargin require intracellular Ca²⁺ either from extracellular sources or from internal stores, that CaMKII plays an important role in the regulation of contraction, that CaMKII acts upstream of MAP kinase to control its activation, and that the MAP kinase signaling pathway can play a significant role in mediating the cellular effects of these Ca²⁺-mobilizing agonists.