DYNORPHIN GENE-EXPRESSION AND RELEASE IN THE MYOCARDIAL-CELL

The expression of the prodynorphin gene was investigated in adult cult ured rat ventricular cardiac myocytes by using a sensitive solution hy bridization RNase protection assay for the quantitative analysis of pr odynorphin mRNA. Myocyte culture in high KCL resulted, after 4 h, in a marked increase in cellular prodynorphin mRNA, while a KCl treatment for 6, 12, or 24 h progressively down-regulated the levels of prodynor phin mRNA below the control value. Immunoreactive dynorphin B, a biolo gically active end product of the precursor, was found to be present i n the culture medium in significantly higher amounts than in the cardi ac myocytes. The levels of this biologically active K opioid receptor agonist significantly increased after 4 h of KCl treatment and were ma rkedly reduced following a 24-h exposure of the cardiac myocytes to KC l. These KCl-induced effects were all abolished by cell incubation in the presence of the calcium channel blocker verapamil. In single cardi ac myocytes, acute stimulation of K opioid receptors with dynorphin B or with the selective agonist U-50,488H increased the level of cytosol ic calcium. This effect was abolished by the specific K opioid recepto r antagonist (Mr-1452) and was not affected by the removal of calcium from the bathing medium. These results suggest that an opioid gene may influence the myocardial function in an autocrine or paracrine fashio n.