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Comparison of Ca²⁺ release and uptake characteristics of the sarcoplasmic reticulum in isolated horse and rabbit cardiomyocytes

¹Institute of Comparative Medicine, University of Glasgow Veterinary School, University of Glasgow, Glasgow G61 1QH; and ²Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom

Submitted 22 January 2004 ; accepted in final form 23 April 2004

Both the cardiac action potential duration (APD) (0.6–1 s) and resting heart rate (30–40 beats/min) in the horse are significantly different from humans and smaller mammals, including the rabbit. This would be anticipated to have consequences for excitation-contraction (EC) coupling and require adaptation of the individual processes involved. The sarcoplasmic reticulum (SR) is one of the main components involved in EC coupling. This study examines and compares the activity of this organelle in the horse with that of the rabbit. In particular, the study focuses on SR Ca²⁺ release via the Ca²⁺ release channel/ryanodine receptor (RyR2) and Ca²⁺ uptake via the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) pump. Isolated cardiomyocytes from both horse and rabbit hearts were permeabilized, bathed in a mock intracellular solution, and exposed to a specified [Ca²⁺]. Rabbit cardiomyocytes exposed to 260 nM [Ca²⁺] produced spontaneous Ca²⁺ release and propagated Ca²⁺ waves. Horse cells failed to produce Ca²⁺ waves; instead, only local release in the form of Ca²⁺ sparks was evident. However, at 550 nM [Ca²⁺], Ca²⁺ waves were produced in both species. Ca²⁺ waves were four times less frequent yet 1.5 times greater in amplitude in the horse compared with the rabbit. Ca²⁺ wave velocity was comparable between the species. The reason for this disparity in Ca²⁺ wave characteristics is unknown. Separate measurements of oxalate-supported Ca²⁺ uptake into the SR suggest that both horse and rabbit cardiomyocytes have comparable levels SERCA activity. The possible reasons for the observed differences in SR Ca²⁺ release between the horse and rabbit are discussed.

cardiac; waves; equine

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