NITRIC-OXIDE HYPERPOLARIZES RABBIT MESENTERIC-ARTERIES VIA ATP-SENSITIVE POTASSIUM CHANNELS

1. Nitric oxide (NO) relaxes vascular smooth muscle (VSM) by mechanisms which are not fully understood. One possibility is that NO hyperpolarizes membranes, thereby diminishing Ca2+ entry through voltage-dependent Ca2+ channels. In the current study, the effects of NO on membrane potential of rabbit mesenteric arteries were recorded using intracellular microelectrodes. 2. NO, released by 3-morpholinosydnonimine (SIN-1, 3 mu M), reversibly hyperpolarized arteries by -9.5 +/- 4.0 mV (means +/- S.D., n = 97) from a resting membrane potential of -53.1 +/- 5.7 mV. The hyperpolarization was blocked by oxyhaemoglobin (20 mu M), and only occurred in arteries pre-treated with N-omega-nitro-L-arginine (100 mu M) or denuded of endothelium. 3. The effect of SIN-1 was concentration dependent (EC(50) approximate to 0.4 mu M), and its dose response was shifted to the left by zaprinast (100 mu M), an inhibitor of cGMP-specific phosphodiesterases. 4. The hyperpolarization due to SIN-1 was modified by changes in extracellular K+ concentration, but not by changes in Ca2+, Na+ or Cl-. The hyperpolarization was blocked by glibenclamide (IC50 approximate to 0.15 mu M), but not by apamin (3-300 nM), barium (5-150 mu M), tetraethylammonium (0.1-10 mM), or 4-aminopyridine (5-500 mu M). The hyperpolarization due to lemakalim (0.03-3 mu M), an activator of ATP-sensitive potassium channels (K-ATP), displayed the same sensitivities to these K+ channel blocking agents, whereas the endothelium-derived hyperpolarizing factor, triggered by the addition of acetylcholine (3 mu M), caused a hyperpolarization (-15.3 +/- 6.2 mV) that was blocked by apamin, but not by any other agent. 5. These results suggest that NO hyperpolarizes VSM in rabbit mesenteric arteries by activating K-ATP channels, with the accumulation of cGMP as an intermediate step.