EFFECTS OF A PHYSIOLOGICAL INSULIN CONCENTRATION ON THE ENDOTHELIN-SENSITIVE CA2-ARTERY SMOOTH-MUSCLE( STORE IN PORCINE CORONARY)

The effect of insulin to attenuate the Ca2+ and contractile response o f vascular smooth muscle to a number of agonists has been described pr eviously, but the Ca2+ regulatory mechanisms of insulin action remain unclear. We determined the effect of a physiological insulin concentra tion (300 pmol/l) on the Ca2+ response of vascular smooth muscle cells of the porcine right coronary artery to endothelin 1 (ET-1); furtherm ore, we examined the cellular Ca2+ stores affected by insulin (i.e., C a2+ stores releasable by inositol 1,4,5-trisphosphate, caffeine, and i onomycin). We measured the Ca2+ responses of acutely isolated single s mooth muscle cells with the fluorescent Ca2+ indicator Fura-2. Acute i nsulin exposure (20 min) significantly attenuated the Ca2+ response of single smooth muscle cells to 10 nmol/l ET-1, This inhibitory effect of insulin was observed both in the presence and absence of extracellu lar Ca2+, In contrast with the effects on ET-1-induced Ca2+ responses, insulin did not inhibit the Ca2+ response to 5 mmol/l caffeine, an ag ent that directly releases sarcoplasmic reticulum Ca2+ stores, Insulin was also without effect on the total cellular Ca2+ store released by 1 mu mol/l ionomycin, a Ca2+-transporting ionophore, When ET-1 and caf feine were given in succession, a sizable caffeine-sensitive Ca2+ stor e could be released from insulin-treated cells but not control cells, indicating that the sarcoplasmic reticulum Ca2+ store of insulin-treat ed cells was not depleted by ET-1, Generalized depletion of the sarcop lasmic reticulum Ca2+ store is not one of the cellular mechanisms invo lved in the effect of insulin on coronary smooth muscle; instead, the effect may be due to an inhibitory influence on transmembrane signal t ransduction, such as diminished ET-1-induced inositol 1,4,5-trisphosph ate production or reduced ability of this phosphoinositol to release s tored Ca2+.