IDENTIFICATION OF THE RECEPTOR SUBTYPE RESPONSIBLE FOR ENDOTHELIN-MEDIATED PROTEIN-KINASE-C ACTIVATION AND ATRIAL-NATRIURETIC-FACTOR SECRETION FROM ATRIAL MYOCYTES

Endothelin-1 (ET-1) is a potent stimulator of atrial natriuretic facto r (ANF) secretion from myocardial cells. In heart tissue there are two ET receptor subtypes (ET(A)-R and ET(B)-R), which can be pharmacologi cally distinguished by the ET isopeptides ET-1 and ET-3. However, the identification of the ET-R subtype responsible for the rapid enhanceme nt of ANF release, which occurs within minutes of exposing cardiac myo cytes to ET, has not been investigated. In the present study ET-1 was about 100-fold more potent than ET-3 at stimulating membrane phosphoin ositide hydrolysis, protein kinase C activation, and ANF release from purified primary atrial myocytes. These responses were completely abol ished by BQ123, an ET(A)-R antagonist. Radioligand binding analyses sh owed that competitor peptides displaced I-125-ET-1 binding to atrial m yocyte ET-Rs with a rank order of potency of ET-1 >> BQ123 > ET-3, a c haracteristic ET(A)-R pharmacological profile. While neither ET-1 or E T-3 altered forskolin-stimulated cAMP levels, suggesting the absence o f the ET(B)-R, basal cAMP levels were also unaffected by the ETs. Nort hern analysis using ET-R subtype-specific probes demonstrated that the ETA-R transcript was present in the cultures at levels at least 50-fo ld greater than the ET(B)-R transcript. These findings demonstrate tha t the stimulation of the phosphatidylinositol/protein kinase C pathway , which is required for maximal ET-stimulated ANF release from primary atrial myocytes, is associated with the activation of only the ET(A)- R, thus defining a specific function for an endogenous ET-R in myocard ial cells.