Inositol trisphosphate, calcium and muscle contraction

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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.