Differential actions of charybdotoxin on central and daughter branch arteries of the rabbit isolated ear

1 By use of rabbit isolated perfused intact ears and isolated perfused segments of central and first generation daughter branch ear arteries, we investigated the actions of charybdotoxin (ChTX), a blocker of calcium-activated K+ channels (K-Ca channels), and N-omega-nitro-L-arginine methyl ester (L-NAME) on pressure-flow and diameter-flow relationships. 2 ChTX (1 nM) induced an upwards shift in the pressure-flow curve in the rabbit intact isolated ear preconstricted with 5-hydroxytryptamine (5-HT; 100 nM) with subsequent administration of L-NAME (100 mu M) inducing a further upwards shift. L-NAME itself induced an upwards shift in the pressure-flow curve, but subsequent administration of ChTX was without significant effect. 3 Microangiographic analysis revealed a tendency of ChTX (1 nM) to decrease vessel diameter in the central ear artery (G(0)) with little effect on the first two generations of daughter branch arteries (G(1) and G(2)) in the intact ear. Subsequent addition of L-NAME (100 mu M) did not significantly further decrease vessel diameter in G(0), but did decrease vessel diameter in G(0) and G(1) L-NAME itself showed a tendency to decrease vessel diameter in G(0), G(1) and G(2) vessels with subsequent addition of ChTX being without significant effect. 4 In an isolated G(0) preparation which was preconstricted with 5-HT (100 nM), ChTX (1 nM) caused an upwards shift in the pressure-flow curve which was augmented by subsequent addition of L-NAME (100 mu M). L-NAME (100 mu M) itself caused an upwards shift in the pressure-flow curve but subsequent addition of ChTX (1 nM) had no significant effect. 5 Tn comparison, in an isolated G(1) preparation which was preconstricted with 5-HT (100 nM), ChTX (1 nM) had no significant effect on the pressure-flow curve relative to control, but subsequent addition of L-NAME (100 mu M) caused an upwards shift. L-NAME (100 mu M) itself induced an upwards shift in the pressure-flow curve with subsequent addition of ChTX (1 nM) being without significant effect. 6 ChTX (10 pM-10 nM) caused a concentration-dependent increase in perfusion pressure in isolated G(0) and G(1) preparations at fixed flow rates of 2 ml min(-1) and 0.5 ml min(-1), respectively. These responses were enhanced in the presence of L-NAME (100 mu M) in G(1) but not Go preparations. 7 We conclude that at 1 nM, ChTX exhibits differential actions on central and daughter branch arteries of the intact ear of the rabbit, which are also apparent in the corresponding arteries when studied in isolation. The action of 1 nM ChTX in G(0) vessels may reflect inhibition of either the release or action of nitric oxide as it was blocked in the presence of L-NAME. At higher concentrations of ChTX, there would appear to be a direct constrictor effect on vascular smooth muscle which is apparent in both G(0) and G(1) vessels. This observed heterogeneity could reflect different distributions of K-Ca channels between central and daughter branch arteries at either the endothelial or smooth muscle levels, or both.