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 relaxati on induced by vasoactive intestinal peptide (VIP) in medial strips of the porcine coronary artery, by determining the effect on the cytosoli c Ca2+ concentration ([Ca2+](i)), the [Ca2+](i)-force relation and the involvement of G-protein. 2 Front-surface fluorometry of fura-2 revea led that U46619, a thromboxane A(2) analogue, and the high K+-depolari zation induced increases in both the [Ca2+](i) and force of the medial strips. At a steady state of contraction, the extent of an increase i n [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) ind uced 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 mu ch greater than those with 30 mM K+-depolarization. 3 The VIP-induced decreases in the [Ca2+](i) and force were attenuated by K+ channel blo ckers such as tetrabutylammonium (TBA: non-selective K+ channel blocke r), charybdotoxin (large conductance Ca2+-activated K+ channel blocker ), and 4-aminopyridine (4-AP: voltage-dependent K+ channel blocker). H owever, neither glibenclamide (ATP-sensitive K+ channel blocker) nor a pamin (small conductance Ca2+-activated K+ channel blocker) had any si gnificant inhibitory effect. 4 In the 30 mM K+-depolarized strips, pre treatment with thapsigargin, a specific Ca2+-ATPase inhibitor of the C a2+ store sites, completely abolished the VIP-induced decrease in [Ca2 +](i), but partially attenuated the VIP-induced decrease in force. 5 V IP shifted the [Ca2+](i)-force relation of the U46619-induced contract ions to the right in a concentration-dependent manner. In the alpha-to xin-permeabilized strips, VIP decreased the force development at a con stant [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 mechan isms: (1) a decrease in [Ca2+](i) by inhibiting the Ca2+ influx presum ably through the membrane hyperpolarization mediated by the activation of the large conductance Ca2+-activated (charybdotoxin-sensitive) Kchannels and voltage-dependent (4-AP-sensitive) K+ channels; (2) a dec rease in [Ca2+](i) by sequestrating cytosolic Ca2+ into thapsigargin-s ensitive Ca2+ store sites; and (3) a decrease in the Ca2+-sensitivity of the contractile apparatus through the activation of G-protein.