Involvement of the protein kinase C system in calcium-force relationships in ferret aorta

Abstract

1 Intracellular Ca²⁺-force relationships were investigated in ferret aortic smooth muscle by the simultaneous measurement of aequorin luminescence and isometric force. Complete calcium-force curves were constructed by plotting calibrated aequorin luminescence versus force, while intracellular [Ca²⁺] was made to change by increasing degrees of K⁺ depolarization or decreasing extracellular [Ca²⁺]. 2 The steady state calcium-force curve in response to K⁺ depolarization exhibited maximal force generation at an intracellular [Ca²⁺] of approximately 4 × 10⁻⁷ m. Further increases in intracellular [Ca²⁺] did not yield additional increments in force. 3 Protein kinase C activation with the phorbol ester, 12-deoxyphorbol-13-isobutyrate 20 acetate (DPBA) produced contractions accompanied by no detectable increases in aequorin luminescence. DPBA significantly shifted the control [Ca²⁺]-force relationship leftward to lower intracellular [Ca²⁺] with an increase in the magnitude of maximal generated force. 4 In aorta maximally precontracted by K⁺ depolarization, the addition of DPBA resulted in a significant increase in force in the absence of further increases in intracellular [Ca²⁺]. Conversely, in muscles maximally precontracted with DPBA, responses to K⁺ depolarization resulted in subsequent increases in force in the presence of simultaneous sustained increases in intracellular [Ca²⁺]. 5 The relatively specific protein kinase C antagonist H-7 caused a significant decrease in intrinsic myogenic tone in the absence of any statistically significant decrease in intracellular [Ca²⁺]. 6 These results suggest that protein kinase C may be an important regulator of vascular smooth muscle contractility by: (1) providing a mechanism by which the apparent [Ca²⁺] sensitivity of the contractile apparatus during agonist-induced contractions is increased, and (2) maintaining intrinsic myogenic tone by a mechanism the [Ca²⁺] requirement of which is satisfied by the resting [Ca²⁺]_i.