ARACHIDONIC-ACID AND ENDOTHELIN POTENTIATE CA2+ TRANSIENTS IN RAT CARDIAC MYOCYTES VIA INHIBITION OF DISTINCT K+ CHANNELS

The release of arachidonic acid by phospholipases in response to cell surface receptor activation may be an important step in the initiation of inotropic events in cardiac muscle. Endothelin has been shown to a ctivate phospholipase A2 and release arachidonic acid in isolated rat hearts. Endothelin also has a positive inotropic effect in cardiac mus cle, suggesting that endothelin increases Ca2+ influx or the amount of Ca2+ released from the sarcoplasmic reticulum. We used suspensions of adult rat ventricular myocytes loaded with fura-2/AM to compare the e ffects of arachidonic acid and endothelin on Ca2+ transients evoked by extracellular ATP. We showed recently (Damron, D. S., and Bond, M. (1 993) Circ. Res. 72, 376-386) that pretreatment of cardiac myocytes wit h arachidonic acid significantly potentiated the amplitude of the ATP- triggered Ca2+ transient. We now report that endothelin also enhances the ATP-triggered Ca2+ transient and that the effect of the combinatio n of maximal doses of endothelin and arachidonic acid is additive. Nei ther endothelin nor arachidonic acid was found to affect the size of t he sarcoplasmic reticulum Ca2+ store. The potentiating effects of both arachidonic acid and endothelin were sensitive to inhibitors of prote in kinase C. Endothelin was also found to stimulate phospholipase C bu t not phospholipase A2. Application of arachidonic acid to individual cardiac muscle cells resulted in inhibition of the transient outward K + current, whereas application of endothelin inhibited the delayed rec tifier current. These effects of arachidonic acid and endothelin were additive, and both effects could be blocked by the protein kinase C in hibitor, staurosporine. Similarly, staurosporine inhibited endothelin- induced increases in isometric contractions in ventricular papillary m uscle. We conclude that arachidonic acid and endothelin may be involve d in the modulation of inotropic activity in cardiac muscle by means o f protein kinase C-dependent inhibition of two distinct K+ channels. T his would result in a prolongation of action potential duration and th us an increase in Ca2+ influx across the sarcolemma.