Ca2+-activated ryanodine binding: mechanisms of sensitivity and intensity modulation by Mg2+, caffeine, and adenine nucleotides

• Published in: Molecular pharmacology Vol. 31; no. 3; pp. 232 - 238
• Main Authors: PESSAH, I. N, STAMBUK, R. A, CASIDA, J. E
• Format: Journal Article
• Language: English
• Published: Bethesda, MD American Society for Pharmacology and Experimental Therapeutics 01.03.1987
• Subjects: Adenine Nucleotides - pharmacology; Alkaloids - metabolism; Animals; Biological and medical sciences; Caffeine - pharmacology; Calcium - metabolism; In Vitro Techniques; Inositol 1,4,5-Trisphosphate; Inositol Phosphates - pharmacology; Intracellular Membranes - metabolism; Ion Channels - metabolism; Kinetics; Magnesium - pharmacology; Medical sciences; Muscle; Pharmacology. Drug treatments; Rabbits; Receptors, Cholinergic - metabolism; Ryanodine - metabolism; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum - metabolism; Calcium; Rabbit; Ionic channel; Lagomorpha; Striated muscle; In vitro; Site of action; Vertebrata; Biological fixation; Mammalia; Sarcoplasmic reticulum; Animal; Affinity; Magnesium; Adenine nucleotide; Caffeine; Biological receptor
• ISSN: 0026-895X

The Ca2+-ryanodine receptor complex is a functional unit at the terminal cisternae (TC) of the sarcoplasmic reticulum (SR) whose proteins comprise the Ca2+ release channels which may be involved in excitation-contraction coupling. Ca2+, Mg2+, caffeine, and adenine nucleotides, but not inositol 1,4,5-trisphosphate, may exert their inotropic effects on skeletal muscle SR by direct allosteric modulation of the [3H]ryanodine-binding site. Micromolar Ca2+ is primarily responsible for activating [3H]ryanodine binding by regulating receptor site density, affinity, and cooperativity. Mg2+ reduces the sensitivity to Ca2+ activation by directly competing with Ca2+ for the activator site. However, inhibition by Mg2+ is overcome in the presence of beta,gamma-methyleneadenosine 5'-triphosphate (AMP-PCP; 1 mM) or caffeine (20 mM). Caffeine dramatically increases the affinity of the Ca2+ activator site for Ca2+, whereas AMP-PCP or cAMP enhances the gating efficiency or the lifetime of the open state of the TC SR channel. A kinetic model is proposed for four functional domains of the Ca2+-ryanodine receptor complex: the Ca2+-regulatory domain which binds Ca2+ with microM affinity is primarily responsible for gating the Ca2+ channel of the TC SR in a cooperative manner, and is inhibited by mM Mg2+ by direct competition for the activator site which appears to contain critical sulfhydryl groups; a Ca2+-activate alkaloid binding domain in close proximity to the channel which binds ryanodine with nM affinity and rapidly occludes upon complex formation; a domain which binds caffeine with low (greater than mM) affinity and directly influences the sensitivity of the Ca2+-regulatory site; and a domain which binds adenine nucleotides with intermediate affinity (less than mM), does not require phosphorylation, and intensifies the Ca2+ signal which triggers opening of the Ca2+-release channel.