Roles of the Sarcoplasmic/Endoplasmic Reticulum Ca-ATPase, Plasma Membrane Ca-ATPase and Na/Ca Exchanger in Regulation of Heart Rate in

ABSTRACT

We investigated the roles of three regulatory proteins that impact [Ca²⁺]_i within cardiac myocytes of Drosophila melanogaster. The NCX (Na⁺/Ca²⁺ exchanger), PMCA (plasma membrane Ca²⁺-ATPase) and SERCA (sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase) were compromised by ionic, pharmacological, mutationalmanipulation, and with a combination of approaches, while heart rate (HR) was monitored. A decrease in SERCA function reduced HR more for intact larva in comparison to a dissected larva. Dissected preparations were used to expose the heart directly to agents. A compromised PMCA also reduced HR; however, attenuated NCX function by low [Na⁺]○ increased HR. KBR7943, a blocker of Ca²⁺ entry via NCX, exposure increased HR. A combined loss of function in all three channels did not show a significant change in HR. The results indicate that NCX and PMCA are important in regulating HR, whereas SERCA does not have as pronounced role for dissected preparations. However, with intact preparations the loss of SERCA function by a mutation does have a significant impact on HR. Pharmacological approaches to alter PMCA and SERCA paralleled the results obtained by ionic and mutational approaches. To further understand the pacemaker activity, intracellular recordings were obtained. Mapping of action-potentials in myocytes revealed that the caudal region of the heart has large amplitude potentials and is likely to contain the pacemaker cells. The Drosophila heart can serve as a genetic model in understanding regulation of ionic currents for pacing cells of various types.

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