HIGHER CA2+ SENSITIVITY OF TRITON-SKINNED GUINEA-PIG MESENTERIC MICROARTERIES AS COMPARED WITH LARGE ARTERIES

Small mesenteric resistance arteries and the main branch of the mesenteric artery (outer in situ diameter 115 +/- 3-mu-m [n = 76] and > 1,000-mu-m, respectively) were skinned with 1% Triton X-100. Both preparations were mounted as rings for circumferential force measurement in an EGTA solution (free Ca2+, < 10 nM; calmodulin, 0.3-mu-M; pH 6.7). Force-pCa curves were obtained by increasing free Ca2+ (0.05 to 30-mu-M). The resulting dose-dependent contractions, after normalization to maximal force development (arteries, 21.4 +/- 2.4 [n = 3]; arterioles, 15.3 +/- 2.1 mN/mm2 [n = 5]) were fitted to sigmoidal force-pCa curves. Values of ED50 and of the cooperativity factor h were 6.08 and 2.39 in arterioles and 5.64 and 1.64 in arteries. The higher Ca2+ sensitivity of arteriolar preparations remained at pH 7.0 at higher calmodulin concentrations and after inhibition of smooth muscle phosphatase with okadaic acid. Total myosin light chain kinase activity in crude arteriolar extracts (using [gamma-P-32]ATP and isolated gizzard light chains as substrates) was approximately 25% of arterial kinase. Both kinase preparations had identical Ca2+ sensitivities. Likewise, total arteriolar phosphatase activity (using P-32-labeled gizzard light chains) was approximately 25% of the arterial activity; both phosphatases had an identical sensitivity toward okadaic acid. The ratio of kinase/phosphatase activities was identical in both tissues. Extracts of both tissues contained two isozymes of the myosin heavy chain as determined with sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The ratio of the more mobile isozyme to the less mobile form changed from 0.41 +/- 0.03 in arteriolar tissues to 0.58 +/- 0.01 (n = 4) in arterial tissues. This study showed a fundamental change of contractile protein regulation with narrowing vascular diameters that is not explained by changes of the key enzymes regulating contraction-relaxation. This leaves the possibility of myosin heavy chain participation in the regulation of Ca2+ sensitivity.