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Cardiac contractile cells depend on calcium in order to function. Understanding the regulation of calcium influx, efflux, and release from the sarcoplasmic reticulum is essential. The focus of this investigation is to address how a reduction of functional Ca²⁺-activated K⁺ (K_Ca) channels, via a mutational line, might impact the heart rate in larva when the SER is also modulated through Ca²⁺ loading and stimulation. The larval heart tube is exposed in situ and flushed with saline. With a known saline composition, a potential therapeutic pharmacological agent, 2-Aminoethyl diphenylborinate (2-APB), was examined for its effect on heart rate, as well as to determine the contribution from K_Ca channels. In this study, it was determined that mutation in the K(Ca) channel (i.e., <i>Slo</i>) showed a different trend than the wild-type CS strain. Exposure to high concentrations of 50 µM 2-APB decreased heart rate in the <i>Slo</i> strain and increased it in the wild-type CS strain. Serotonin increased heart rate in both thapsigargin- and 2-APB-treated larvae, with no significant difference between the strains.