Role of EDHF in the vasodilatory effect of loop diuretics in guinea-pig mesenteric resistance arteries

1 Relaxing effect of loop diuretics, piretanide and furosemide in compariso n with acetylcholine (ACh) was investigated in guinea-pig isolated mesenter ic resistance arteries. 2 Concentration-response curves to ACh (0.001-10 muM) and diuretics (0.0001 -1 muM) were constructed in noradrenaline (10-30 muM)-precontracted arterie s incubated either in normal physiological salt solution (PSS) or in 30 mM KCI PSS (K-PSS). 3 In PSS, maximal relaxations (R-max) and pD(2) to ACh were 87 +/- 2% and 7 .1 +/- 0.1 (n = 10). L-N-G-nitro-arginine methyl eater (L-NAME, 100 muM) re duced R-max by 20% (P < 0.01, n = 7) and pD(2) by 10% (P < 0.01). In contra st, indomethacin (10 muM) increased R-max by 19% (P < 0.01, n = 8) and pot by 10% (P < 0.05). Combination of L-NAME + indomethacin reversed the effect observed with either of these inhibitors used alone. In K-PSS, R-max was a ttenuated by 40% (P < 0.001, n = 6) compared to PSS. L-NAME reduced R-max b y 65% (P < 0.01, n = 5) and increased pot by 15 fold. L-NAME + indomethacin suppressed the resistant relaxation. 4 In PSS + L-NAME + indomethacin, inhibitors of small (SKCa,; apamin, 0.1 m uM) and large (BKCa,; iberiotoxin and charybdotoxin, 0.1 muM) conductance C a2+-sensitive K--channels used alone had little effect on the ACh-response. Combination of apamin + iberiotoxin reduced R-max by 40% (P < 0.05, n = 7) while apamin + charybdotoxin fully abolished the resistant relaxation. 5 In PSS, piretanide and furosemide induced relaxation with R-max 89 +/- 3% vs 84 +/- 5% and pD(2): 8.5 +/- 0.1 vs 7.7 +/- 0.2 (P < 0.01) for piretani de (n = 11) and furosemide (n = 10), respectively. Endothelial abrasion sup pressed relaxation to diuretics. L-NAME and indomethacin used alone or in c ombination did not significantly modify the response to diuretics. 6 In K-PSS, piretanide-induced relaxation was abolished whereas that to fur osemide was reduced by 70% (P < 0,001, n = 9) compared to PSS and was suppr essed by L-NAME + indomethacin. In PSS + L-NAME + indomethacin, apamin slig htly reduced relaxation to diuretics whereas charybdotoxin or iberiotoxin a bolished the response. 7 These results indicate that ACh-evoked relaxation is mediated by both NO/ PGl(2)-dependent and -independent mechanisms. The EDHF-dependent component relies on activation of Ca2+-activated K+ channels, is sensitive to a combi nation of apamin + charybdotoxin and to a smaller degree to a combination o f apamin + iberiotoxin, Loop diuretic-induced relaxation is endothelium-dep endent. appears to be mediated by NO, PGl(2) and EDHF for furosemide and ED HF only for piretanide. For the two diuretics, opening of BKCa, channels ma y be involved in the relaxation.