Comparison between alpha-adrenergic- and K-opioidergic-mediated inositol(1,4,5)P₃/inositol(1,3,4,5)P₄ formation in adult cultured rat ventricular cardiomyocytes

In adult cultured rat ventricular cardiac myocytes, both the α-adrenergic agonist phenylephrine and the selective K opioid receptor ligand U-50,488H affected phosphoinositide turnover. Phenylephrine, over a time course of 10 min, caused a transient increase in Ins(1,4,5)P₃ which peaked at 1 min and had returned to control at 2 min. In addition, phenylephrine produced a progressive and sustained increase in the formation of Ins(1,3,4, 5)P₄ which achieved a plateau after 5 min of exposure to the agonist. U-50,488H induced an increase in Ins(1,4,5)P₃ which peaked at 1 min at a level significantly higher than that due to phenylephrine and was still elevated after 10 min exposure to the K opioid receptor agonist. In addition, U-50,488H caused a sustained increase in Ins(1,3,4,5)P₄ which was comparable to that due to phenylephrine. The stimulatory effects produced by phenylephrine and U-50,488H were receptor-mediated events, since they were fully antagonized by their respective antagonists, phentolamine or Mr-1452.