Contraction muscle studies, sensitivity

An initial hint that Ca2+ stores are present and functional in smooth muscle cells came from earlier experiments revealing that agonist-induced contractions could be observed in the absence of extracellular Ca2+. It is now known that smooth muscle Ca2+ stores express two types of Ca2+ release channels, the ryanodine receptor (RyR) and the inositol-1,4,5-trisphosphate (L1SP3) receptor (L1SP3R) (Somlyo Somlyo 1994). Recent studies have shown that Ca2+ release from intracellular Ca2+ stores plays various important roles in the regulation of smooth muscle contraction. Local and transient releases of Ca2+ from RyR near the surface membrane, which are called Ca2+ sparks, activate Ca2+-sensitive K+... 

The presence of proctolin in the reproductive tract of a variety of insects and the action of this peptide on such tissues has been a current focus for many studies. The visceral muscles of the oviducts of Locusta migratoria. for example, are remarkably sensitive to proctolin (30)- Amounts of the peptide as low as 2 fmol induce a tonic contraction that is dose-dependent up to 200 fmol. A proctolin-like immunoreactive substance was found in the axons of nerves leading to the oviduct as well as in a number of cell bodies in the Vllth abdominal ganglion. Moreover, reverse-phase HPLC of tissue extracts of oviducts, oviducal nerves, and the Vllth abdominal ganglia indicate that the proctolin-like bioactive substance co-elutes with authentic proctolin. The peptide was present in areas of the oviduct that receive extensive innervation while it was ten-fold less in areas that receive little or no innervation. 

Study of the molecular biology of calcium regulation of muscle contraction was initiated by the discovery of a new protein factor sensitizing actomyosin to calcium ions (Ebashi, 1963 Ebashi and Ebashi, 1964). This protein factor was called native tropomyosin, because of its similarity in amino acid composition to tropomyosin, which had been discovered earlier (Bailey, 1946, 1948). It was soon found that this factor is a complex of tropomyosin and a new globular protein, termed troponin (Ebashi and Kodama, 1965 Ebashi et al., 1968). Thus four proteins, i.e., myosin, actin, troponin, and tropomyosin, are involved in calcium-regulated physiological muscle contraction (Ebashi et al., 1968, 1969 Ebashi and Endo, 1968). The contractile interaction between myosin and actin is depressed by troponin and tropomyosin in the absence of calcium ions. When calcium ion acts on troponin, this depression is removed and the contractile interaction is then activated (Figs. 1 and 2). 

Several authors have concluded that PKC plays no major role in smooth muscle contraction (Colburn et al., 1988 Sutton and Haeberle, 1990 Hartshorne and Kawamura, 1992). On the other hand, two other studies suggest that activation of PKC in vascular smooth muscle increases the Ca + sensitivity of contraction (Jiang effll., 1994 MasuoefaZ., 1994). Accordingly, the significance of PKC in smooth muscle remains unsettled. The role of PKC in smooth muscle is further discussed in Chapters 12, 21, 24, and 26 (this volume). 

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