Inositol trisphosphate, calcium and muscle contraction

Abstract

The identity of organelles storing intracellular calcium and the role of Ins(1,4,5)P₃, in muscle have been explored with, respectively, electron probe X-ray microanalysis (EPMA) and laser photolysis of ‘caged’ compounds. The participation of G-protein(s) in the release of intracellular Ca²⁺was determined in saponin-permeabilized smooth muscle. The sarcoplasmic reticulum (SR) is identified as the major source of activator Ca²⁺in both smooth and striated muscle; similar (EPMA) studies suggest that the endoplasmic reticulum is the major Ca²⁺storage site in non-muscle cells. In none of the cell types did mitochondria play a significant, physiological role in the regulation of cytoplasmic Ca²⁺. The latency of guinea pig portal vein smooth muscle contraction following photolytic release of phenylephrine, an α₁-agonist, is 1.5 ± 0.26 s at 20 °C and 0.6 ± 0.18 s at 30 °C; the latency of contraction after photolytic release of Ins(1,4,5)P₃from caged Ins(1,4,5)P₃is 0.5 ± 0.12 s at 20 °C. The long latency of α₁-adrenergic Ca²⁺release and its temperature dependence are consistent with a process mediated by G-protein-coupled activation of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P₂) hydrolysis. GTPγS, a non-hydrolysable analogue of GTP, causes Ca²⁺release and contraction in permeabilized smooth muscle. Ins(1,4,5)P₃has an additive effect during the late, but not the early, phase of GTPγS action, and GTPγS can cause Ca²⁺release and contraction of permeabilized smooth muscles refractory to Ins(l,4,5)P₃. These results suggest that activation of G protein (s) can release Ca²⁺by, at least, two G-proteinregulated mechanisms: one mediated by Ins(1,4,5)P₃and the other Ins(1,4,5)P₃- independent. The Ins(1,4,5)P₃, 5-phosphatase activity and the slow time-course (seconds) of the contractile response toIns(1,4,5)P₃released with laser flash photolysis from caged Ins(1,4,5)P₃in frog skeletal muscle suggest that Ins(1,4,5)P₃is unlikely to be the physiological messenger of excitation-contraction coupling of striated muscle. In contrast, in smooth muscle the high Ins Ins(1,4,5)P₃-5-phosphatase activity and the rate of force development after photolytic release of Ins(1,4,5)P₃are compatible with a physiological role of Ins(1,4,5)P₃as a messenger of pharmacomechanical coupling.