[New trends in studying the regulatory mechanism of smooth muscle contraction].
In this paper, we briefly review current topics about smooth muscle with regards to Ca2+ release, Ca2+ sensitization, and Ca2+ regulation of contraction. Inositol 1,4,5-trisphosphate releases Ca2+ from the sarcoplasmic reticulum, where Ca(2+)-dependent immediate feedback control may work. However, the involvement of this feedback control in the Ca(2+)-induced Ca2+ release mechanism remains to be elucidated. Either agonist or GTP gamma S is known to increase the Ca2+ sensitivity of myofilaments. The agonist-induced Ca2+ sensitization could be explained by the up-regulation due to myosin light chain kinase or by the down-regulation due to myosin light chain phosphatase. The GTP gamma S-induced Ca2+ sensitization seems to be mediated by rho A p21, a small G protein. Thus, myosin phosphorylation is not the obligatory way to regulate the actin-myosin interaction. We propose that cross-linking between actin and myosin may work as an alternative way to regulate the interaction from biochemical studies. The candidates for the cross-linkers are caldesmon, calponin and myosin light chain kinase. The inhibitory effect of Ca2+ on the interaction, which is observed under the specific conditions for measuring smooth muscle contraction, may hold the key to finding the physiological significance of the cross-linking activity.
Saida
K
K
Department of Pharmacology, Gunma University School of Medicine, Maebashi, Japan.
Kohama
K
K
D000199:Actins / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D015195:Drug Design; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D009219:Myosin-Light-Chain Kinase / Q000378:metabolism; D009218:Myosins / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism
D000199:Actins; D015544:Inositol 1,4,5-Trisphosphate; D009219:Myosin-Light-Chain Kinase; D009218:Myosins; D002118:Calcium