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

Abstract

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 μM Ca²⁺ were studied with respect to maximal isometric tension (P_o) and rate of contraction (T_0.5C), and following relaxation in 18 nm Ca²⁺, the rate of relaxation (T_0.5R). Acidic pH (pH 6.50) decreased P_o to 87% of isometric force obtained at pH 6.90, and increased the rate of contraction as shown by a decrease of T_0.5C to 80%. In contrast, T_0.5R 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<0.01) shift in pCa₅₀ between pH 6.70 ([Ca²⁺]=0.46 μM) and pH 7.10 ([Ca²⁺]=0.87 μM). 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 T_0.5R, suggesting the difference between relaxation rates was not due to Ca²⁺ diffusion limitations from the skinned strip. We suggest pH changes can after the contractile and relaxation responses in vascular smooth muscle and these effects may be related to LC₂₀ phosphorylation/dephosphorylation regulatory mechanisms.