Inositol 1,4,5-trisphosphate receptor expression in cardiac myocytes.

Abstract

Abstract. 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 Ca 2+ 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 Ca2+-induced Ca 2+ release via the RYR, a role for IP3-mediated Ca 2+ 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 ,,o50-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.

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

“…This converts to ∼1.5−3.5 million RyRs in a typical ∼30 pl ventricular myocyte [53]. Direct comparison of the expression of RyRs and IP 3 Rs in ventricular myocytes revealed that RyRs outnumber IP 3 Rs by a factor between 50:1 and 100:1 [54]. Microsomal preparations from hearts of various species, including human, exhibit a much larger density of RyRs than IP 3 Rs.…”

The inositol 1,4,5-trisphosphate receptor is essential for T-cell receptor signaling.

“…A second anti-IP3R antibody raised against a synthetic peptide corresponding to amino acid residues 2652-2663 of the type 1 IP3R gave identical results. Both antibodies were raised in rabbits as described (9). Permeabilized (100% methanol for 5 min at room temperature) or nonpermeabilized Jurkat cells were treated with affinitypurified anti-IP3R antibody at 1:10,000 dilution.…”

Regulation of ryanodine receptor-dependent calcium signaling by polycystin-2

“…Both Pkd2 and RyR2 are expressed in mouse cardiomyocytes starting at embryonic day 9.5 (28,30). Although the InsP 3 R type 2 is present in the cardiomyocytes, its mRNA was shown to be 50 times lower than that of RyR2 in cardiomyocytes (31,32). To further explore the regulation of RyR2 function by PC2, we examined the frequency of spontaneous Ca 2ϩ oscillations of the Pkd2 ϩ/ϩ and Pkd2 Ϫ/Ϫ cardiomyocytes isolated at embryonic day 17.5.…”