Phorbol Ester-induced Potentiation of Myogenic Tone is not Associated with Increases in Ca2+ Influx, Myoplasmic Free Ca2+ Concentration, or 20-kDa Myosin Light Chain Phosphorylation

We studied the extent of Ca2+ influx, myoplasmic free Ca2+ concentration changes, and phosphorylation of the regulatory 20-Kda myosin light chain (LC20) associated with the potentiation of stretch-induced myogenic tone in the rabbit facial vein. The protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA), was used to augment Ca2+-dependent stretch-induced myogenic tone. Veins stretched to an optimal resting tension in physiological salt solution (PSS) containing 0.4 mM of Ca2+ developed stretch-induced myogenic tone. Tissues incubated in Ca2+-free PSS, either with or without PMA (0.1 μM) did not develop myogenic tone. Readmission of Ca2+ (0.4 mM) caused three-fold increase in the contraction in PMA-treated segments (710 ± 60 mg, n = 29 v control: 188 ± 10 mg, n = 24). This increased contraction was not associated with additional increases in either Ca2+ influx (73.5 ± 6.9 pmol/mg of tissue/min, n = 29 v control: 61.1 ± 5.7 pmol/mg of tissue/min, n = 24), myoplasmic free Ca2+ concentration of LC20, (0.44 ± 0.02 mol PO4/mg LC20, n = 9 v control: 0.43 ± 0.03 mol PO4/mol LC20, n = 7). Our results suggest that PKC activation amplifies stretch-induced myogenic tone in the rabbit facial vein through target proteins that are not associated with regulation of Ca2+ influx, myoplasmic free Ca2+ concentration, or LC20 phosphorylation. We conclude that the PKC-mediated potentiation of stretch-induced myogenic tone is due to an increased sensitivity of the contractile apparatus to Ca2+.