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NEUROHUMORAL CONTROL OF CARDIOVASCULAR FUNCTION

Temperature and Ca²⁺ dependence of [³H]ryanodine binding in the burbot (Lota lota L.) heart

Department of Biology, University of Joensuu, Joensuu, Finland

Submitted 23 June 2005 ; accepted in final form 15 September 2005

Opening and closing of the cardiac ryanodine (Ry) receptor (RyR) are coordinated by the free intracellular Ca²⁺ concentration, thus making the Ca²⁺ binding properties of the RyR important for excitation-contraction coupling. Unlike mammalian cardiac RyRs, which lose their normal function at low temperatures, RyRs of ectothermic vertebrates remain operative at 2–4°C, as indicated by Ry sensitivity of contractile force. To investigate the mechanisms of low temperature adaptation of ectothermic RyRs, we compared Ca²⁺-dependent kinetics of [³H]ryanodine binding in cardiac preparations of a fish (burbot, Lota lota) and a mammal (rat). The number of ventricular [³H]ryanodine binding sites determined at 20°C was 1.54 times higher in rat than burbot heart (0.401 ± 0.039 and 0.264 ± 0.019 pmol/mg protein, respectively) (P < 0.02), while the binding affinity (K_d) for [³H]ryanodine was similar (3.38 ± 0.63 and 4.38 ± 1.14 nM for rat and burbot, respectively) (P = 0.47). The high-affinity [³H]ryanodine binding to burbot and rat cardiac preparations was tightly coordinated by the free Ca²⁺ concentration at both 20°C and 2°C and did not differ between the two species. Half-maximal [³H]ryanodine binding occurred at 0.191 ± 0.027 µM and 0.164 ± 0.034 µM Ca²⁺ for rat and at 0.212 ± 0.035 µM and 0.188 ± 0.039 µM Ca²⁺ for burbot (P = 0.65), at 2°C and 20°C, respectively. In two other fish species, rainbow trout (Oncorhynchus mykiss) and crucian carp (Carassius carassius), the Ca²⁺-binding affinity at 20°C was 4.4 and 5.9 times lower, respectively, than in the burbot. At 20°C, the rate of [³H]ryanodine binding to the high-affinity binding site was similar in rat and burbot but was drastically slowed in rat at 2°C. At 2°C, [³H]ryanodine failed to dissociate from rat cardiac RyRs, and at 10°C and 20°C, the rate of dissociation was two to three times slower in rat than burbot preparations. The latter finding is compatible with a channel gating mechanism, where the closing of the Ca²⁺ release channel is impaired or severely retarded by low temperature in rat but less so in burbot preparations. The stronger effect of low temperature on association and dissociation rate of [³H]ryanodine binding in rat compared with burbot suggests that RyRs of the ectothermic fish, unlike those of endothermic rat, are better able to open and close at low temperatures.

ryanodine receptors; fish heart; sarcoplasmic reticulum

This article has been cited by other articles:

				J. Haverinen and M. Vornanen Comparison of sarcoplasmic reticulum calcium content in atrial and ventricular myocytes of three fish species Am J Physiol Regulatory Integrative Comp Physiol, October 1, 2009; 297(4): R1180 - R1187. [Abstract] [Full Text] [PDF]