MYOCARDIAL ALPHA-THROMBIN RECEPTOR ACTIVATION INDUCES HYPERTROPHY ANDINCREASES ATRIAL-NATRIURETIC-FACTOR GENE-EXPRESSION

The protease, alpha-thrombin (alphaTh), affects myocardial cell contra ctility, a feature common among agents that induce hypertrophy. Howeve r, it is not known whether cardiac myocytes possess alphaTh receptors (alphaTh-R), or if long term treatment with alphaTh can enhance growth and gene expression. In the present study primary neonatal rat ventri cular myocytes expressed a 3.6-kilobase mRNA species that hybridized w ith a rat alphaTh-R-specific probe. After 48 h, alphaTh induced hypert rophy, sarcomeric organization, and enhanced atrial natriuretic factor (ANF) expression, all of which were blocked by the alphaTh-selective protease inhibitor, D-Phe-Pro-Arg-chloromethyl ketone. The alphaTh-R a gonist peptide, SFLLRNPND, was a potent activator of ANF expression, h owever, the non-agonist, FLLRNPND, was inactive. Transfection experime nts showed the enhancement of ANF expression by alphaTh to be transcri ptional. The abilities of alphaTh to induce myocyte hypertrophy and to augment ANF transcription and peptide production were inhibited by th e protein kinase C inhibitor, chelerythrine, and by the tyrosine kinas e inhibitor, tyrphostin. Thus, myocardial cell alphaTh-Rs are stimulat ed by the specific proteolytic actions of alphaTh, and pathways involv ing both protein kinase C and protein tyrosine kinases are required fo r subsequent hypertrophy and ANF expression. Further, these findings s uggest a new role for extracellular proteases as regulators of myocard ial cell gene expression and growth.