The mechanisms of the relaxation induced by vasoactive intestinal peptide in the porcine coronary artery

1 This study was designed to investigate the mechanism of the relaxation induced by vasoactive intestinal peptide (VIP) in medial strips of the porcine coronary artery, by determining the effect on the cytosolic Ca2+ concentration ([Ca2+](i)), the [Ca2+](i)-force relation and the involvement of G-protein. 2 Front-surface fluorometry of fura-2 revealed that U46619, a thromboxane A(2) analogue, and the high K+-depolarization induced increases in both the [Ca2+](i) and force of the medial strips. At a steady state of contraction, the extent of an increase in [Ca2+](i) induced by 100 nM U46619 was similar to that induced by 30 mM K+-depolarization. VIP concentration-dependently (1 nM-1 mu M) induced transient decreases in both the [Ca2+](i) and force of the medial strips precontracted with 100 nM U46619. The decreases in the [Ca2+](i) and force induced by VIP during the contraction with U46619 were much greater than those with 30 mM K+-depolarization. 3 The VIP-induced decreases in the [Ca2+](i) and force were attenuated by K+ channel blockers such as tetrabutylammonium (TBA: non-selective K+ channel blocker), charybdotoxin (large conductance Ca2+-activated K+ channel blocker), and 4-aminopyridine (4-AP: voltage-dependent K+ channel blocker). However, neither glibenclamide (ATP-sensitive K+ channel blocker) nor apamin (small conductance Ca2+-activated K+ channel blocker) had any significant inhibitory effect. 4 In the 30 mM K+-depolarized strips, pretreatment with thapsigargin, a specific Ca2+-ATPase inhibitor of the Ca2+ store sites, completely abolished the VIP-induced decrease in [Ca2+](i), but partially attenuated the VIP-induced decrease in force. 5 VIP shifted the [Ca2+](i)-force relation of the U46619-induced contractions to the right in a concentration-dependent manner. In the alpha-toxin-permeabilized strips, VIP decreased the force development at a constant [Ca2+](i) level (pCa = 6.5) in a GTP-dependent manner, which was antagonized by guanosine-5'-O-(beta-thiodiphosphate) (GDP beta S). 6 We thus conclude that VIP relaxes the coronary artery via three mechanisms: (1) a decrease in [Ca2+](i) by inhibiting the Ca2+ influx presumably through the membrane hyperpolarization mediated by the activation of the large conductance Ca2+-activated (charybdotoxin-sensitive) K+ channels and voltage-dependent (4-AP-sensitive) K+ channels; (2) a decrease in [Ca2+](i) by sequestrating cytosolic Ca2+ into thapsigargin-sensitive Ca2+ store sites; and (3) a decrease in the Ca2+-sensitivity of the contractile apparatus through the activation of G-protein.