Mechanisms and their pharmacology of Ca ion mobilization in skeletal, cardiac and smooth muscles are reviewed. In skeletal muscle, it is very likely that depolarization of T-tubule membrane causes conformational changes of dihydropyridine (DHP) receptors in the T-membrane, which in turn, most probably through some kind of protein-protein interaction, open the Ca2+ release channel in the sarcoplasmic reticulum (SR), the ryanodine receptor. Both the DHP receptor and ryanodine receptor have already been purified and sequenced, but the nature of the information transduction between these proteins still remains to be solved. Both of these proteins appear to have dual functions : the DHP receptor as a voltage sensor as described above and as a voltage-dependent Ca2+ channel and the ryanodine receptor as a physiological Ca2+ release channel and as a Ca2+-induced Ca2+ release (CICR) channel, an abnormality of which is known to cause malignant hyperthermia. In cardiac muscle, Ca2+ influx is essential not as the main Ca2+ source but to release Ca2+ from the SR, probably not through the CICR mechanism in the narrow sense but through a mechanism dependent on both Ca2+ and T-tubule depolarization. Several mechanisms of Ca2+ mobilization are used in smooth muscles and their features, different from those in striated muscles, are briefly reviewed.
The molecular basis of opioid receptor mechanisms was studied in reconstitution experiments using purified or membrane-bound opioid receptors and purified GTP-binding proteins (G-proteins). μ-Opioid receptor exclusively purified from rat brains was reconstituted with G-proteins in lipid vesicles. The μ-agonist stimulated the G-protein activity in both G, or Go-reconstituted vesicles. The stoichiometry revealed that one molecule of μ-receptor is functionally coupled to plural numbers of G, or Go molecules and that μ-preceptor exists in at least two different subtypes, μi and μo, separately coupled to Gi and Go, respectively. In addition, when the μ-receptor was phosphorylated by cAMP-dependent protein kinase, the μ-agonist-stimulation of G-protein activity disappeared, while the guanine nucleotide-sensitivity of agonist binding was unchanged. These findings suggest that there are independent domains in the receptor which are related to functional coupling to G-protein and to the agonist-binding modulation by G-protein. κ-Opioid receptor agonist inhibited the G-protein activity in guinea pig cerebellar membranes. Further experiments revealed that the κ-opioid receptor is functionally coupled to an inhibition of phospholipase C activity via an inhibition of Gi-activity. Such a receptor-mediated inhibition of G-protein activity may be the first demonstration of a signal transduction mechanism. The δ-opioid receptor agonist showed no effect on G-protein activity in guinea pig striatal and rat cortical membranes, while it stimulated it in NG108-15 cells. In all these membranes, the δ-agonist binding was markedly reduced by GTPγS in the presence of MgCl2. These findings suggest that δ-receptors in the brain might be coupled to G-protein without signal transduction.