Influence of pH on isometric force development and relaxation in skinned vascular smooth muscle

The effects of pH (from pH values 6.50-7.10) on isometric tension development and relaxation were investigated in Triton X-100 "skinned" rat caudal artery. Helically cut skinned strips contracted in 21 microM Ca2+ were studied with respect to maximal isometric tension (Po) and rate of contraction (T0.5 C), and following relaxation in 18 nm Ca2+, the rate of relaxation (T0.5 R). Acidic pH (pH 6.50) decreased Po to 87% of isometric force obtained at pH 6.90, and increased the rate of contraction as shown by a decrease of T0.5 C to 80%. In contrast, T0.5 R increased 4.5-fold, indicating that with a change of only 0.40 pH units, relaxation rates were dramatically decreased. pCa-tension curves at pH values 6.50, 6.70, 6.90 and 7.10 indicated no significant shift in half maximal activation (pCa50) between pH 6.50 and 6.70, but a significant (P less than 0.01) shift in pCa50 between pH 6.70 [( Ca2+] = 0.46 microM) and pH 7.10 [( Ca2+] = 0.87 microM). Compared to contractions at pH 6.90, myosin light chain (LC20) phosphorylation at pH 6.50 was significantly greater at 30 and 60 s into contraction but not significantly different at 3-10 min. At both pH 6.50 and 6.90, dephosphorylation was rapid and substantially preceded relaxation; LC20 dephosphorylation and relaxation occurred more rapidly at pH 6.90 than at 6.50. At pH 6.50 and 6.90, relax solutions made with increased Ca2+ buffering capacity showed no effect in enhancing T0.5 R, suggesting the difference between relaxation rates was not due to Ca2+ diffusion limitations from the skinned strip.(ABSTRACT TRUNCATED AT 250 WORDS)