Calcium release from intracellular stores is the signal generated by numerous regulatory pathways including those mediated by hormones, neurotransmitters and electrical activation of muscle. Recently two forms of intracellular calcium release channels (CRCs) have been identified. One, the inositol 1,4,5-trisphosphate receptors (IP3Rs) mediate IP3-induced Ca2+ release and are believed to be present on the ER of most cell types. A second form, the ryanodine receptors (RYRs) of the sarcoplasmic reticulum, have evolved specialized functions relevant to muscle contraction and are the major CRCs found in striated muscles. Though structurally related, IP3Rs and RYRs have distinct physiologic and pharmacologic profiles. In the heart, where the dominant mechanism of intracellular calcium release during excitation-contraction coupling is Ca(2+)-induced Ca2+ release via the RYR, a role for IP3-mediated Ca2+ release has also been proposed. It has been assumed that IP3Rs are expressed in the heart as in most other tissues, however, it has not been possible to state whether cardiac IP3Rs were present in cardiac myocytes (which already express abundant amounts of RYR) or only in non-muscle cells within the heart. This lack of information regarding the expression and structure of an IP3R within cardiac myocytes has hampered the elucidation of the significance of IP3 signaling in the heart. In the present study we have used combined in situ hybridization to IP3R mRNA and immunocytochemistry to demonstrate that, in addition to the RYR, an IP3R is also expressed in rat cardiac myocytes. Immunoreactivity and RNAse protection have shown that the IP3R expressed in cardiac myocytes is structurally similar to the IP3R in brain and vascular smooth muscle. Within cardiac myocytes, IP3R mRNA levels were approximately 50-fold lower than that of the cardiac RYR mRNA. Identification of an IP3R in cardiac myocytes provides the basis for future studies designed to elucidate its functional role both as a mediator of pharmacologic and hormonal influences on the heart, and in terms of its possible interaction with the RYR during excitation-contraction coupling in the heart.

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