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 x 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.