Abstract 210: First Molecular Evidence that Inositol Trisphosphate Signaling Contributes to Infarct Size Reduction with Postconditioning
Stimulation of G-protein coupled receptors, followed by production of diacylglycerol (DAG) and activation of one or more isoforms of protein kinase C, has been implicated to play a role in postconditioning (PostC: the reduction of infarct size seen when reperfusion is initiated in a stuttered manner). Production of DAG is, however, accompanied by the ‘in parallel’ generation of the second messenger inositol 1,4,5-trisphosphate (IP3). While IP3 signaling (generation of IP3, followed by binding to IP3 receptors) is well-recognized to participate in regulation of calcium homeostasis, the role of IP3 signaling in PostC-induced cardioprotection is, at present, unknown. To investigate this issue, isolated buffer-perfused hearts were obtained from:
adult ltpr-1opt+/− mice displaying spontaneous mutation of the IP3 receptor-1 gene and, thus, reduced expression of IP3 receptor-1 RNA and protein (heterozygotes were used, as homozygotes die soon after birth); and
age-matched C57BL/6J mice. All hearts underwent 30 min global ischemia and received either standard, abrupt reperfusion (controls) or 3 10-sec cycles of stuttered reflow (PostC).
In addition, in supplemental pilot experiments, the role of IP3 signaling in PostC-induced protection was assessed using a second molecular approach: by assessing the efficacy of PostC in C57 mice previously injected with siRNA (small interfering RNA) targeting the IP3 receptor-1 gene (3 nM/day for 3 days). In C57 mice, infarct size (delineated by tetrazolium staining) was, as expected, reduced in the PostC group vs controls (27 ± 3% vs 53 ± 5% of the LV; p < .05). There were no significant differences in myocyte size, cardiac fibrosis, heart weight/body weight ratio or baseline hemodynamics in ltpr-1opt +/− vs C57 cohorts. However, ltpr-1opt +/− mice:
were characterized by a 50% deficit in expression of IP3 receptor-1 protein; and
were refractory to PostC-induced protection (infarct size: 56 ± 2% vs 58± 8% in mutant control vs PostC groups).
Corroborative results were obtained in siRNA-injected C57 mice: i.e., 50% reduction in IP3 receptor-1 expression and no reduction of infarct size with PostC. Taken together, these data provide novel evidence that IP3 signaling contributes to infarct size reduction with PostC.