The involvement of ATP-sensitive potassium channels in β₂-adrenoceptor agonist-induced vasodilatation on rat diaphragmatic microcirculation

1. The effects of glibenclamide (GLB), a blocker of ATP-sensitive potassium (K(ATP)) channels, on diaphragmatic microcirculation in male Sprague-Dawley rats were assessed under basal conditions and after β₂-adrenoceptor-agonist stimulation. In addition, forskolin was used to bypass β-adrenoceptors and GTP-binding proteins (G-protein) to explore the possible mechanism of GLB effects. For comparison, the relationships between K(ATP) channel activity and cyclic GMP-mediated vasodilator responses to acetylcholine (ACh) and sodium nitroprusside (SNP) were also assessed. 2. Male Sprague-Dawley rats were anaesthetized with urethane and mechanically ventilated. The left hemi-diaphragm of each rat was prepared and microvascular blood flow (Q(LDF)) was recorded with laser-Doppler flowmetry during continuous superfusion with bicarbonate-buffered, prewarmed Ringer solution. The drugs were topically applied to the surface of the hemi-diaphragm. 3. Salbutamol (0.32-32 μM), terbutaline (0.32 μM-0.32 μM) and forskolin (0.32-10 μM) each elicited a concentration-dependent increase in Q(LDF). 4. Baseline microvascular blood flow was unaffected by a 30 min suffusion of 1 μM GLB (295 ± 51 mV vs 325 ± 62 mV, P = 0.738). 5. The vasodilator response elicited by salbutamol (0.32 μM, 1 μM and 3.2 μM), was significantly attenuated by a 30 min superfusion with 1 μM GLB; this salbutamol-induced vasodilatation was mediated via an interaction with β-adrenoceptor receptors, as in other experiments it was greatly inhibited by 30-min superfusion with propranolol (10 μM). 6. Similarly, following 30-min superfusion with GLB (1 μM), the terbutaline (1 μM, 3.2 μM and 10 μM)-induced vasodilator response was almost abolished and the vasodilator responses induced by incremental concentrations of forskolin (0.32 μM, 1 μM and 3.2 μM) were also significantly attenuated. 7. Cromakalim (1.5 μM, 3 μM and 3.2 μM) produced an increase of Q(LDF) in a dose-dependent manner, which was virtually abolished by GLB (1 μM). In contrast, the vasodilator responses induced by acetylcholine (32 μM, 0.1 mM, and 0.32 mM) or sodium nitroprusside (3.2 μM, 10 μM and 20 μM) were independent of GLB (1 μM). 8. In conclusion, K(ATP) channels may be functional, but tonically inactive in the resting diaphragmatic microcirculation and the vasodilator effect of β₂-adrenoceptor agonists may be partly mediated by K(ATP) channels; the activation of K(ATP) channels may involve the accumulation of cyclic AMP in vascular smooth muscle cells.