It has been previously observed that nitric oxide (NO) contributes to hypoxic pial artery dilation and that both sodium nitroprusside (SNP), a releaser of NO, and hypoxia elicit dilation via activation of ATP-sensitive K+ channels in the newborn pig. Other studies, however, have shown that NO activates calcium-sensitive K+ (K(Ca)) channels. The present study, therefore, was designed to investigate the role of K(Ca)-channel activation in NO and hypoxic dilation and to relate this mechanism to the previously observed role of NO in hypoxic dilation in newborn pigs equipped with closed cranial windows. SNP (10(-8) and 10(-6) M) elicited pial artery dilation that was unchanged in the presence of the K(Ca)-channel antagonist iberiotoxin (10(-7) M; 10 +/- 1 and 20 +/- 1 vs. 9 +/- 1 and 20 +/- 2% for 10(-8) and 10(-6) M SNP in the absence and presence of iberiotoxin, respectively). Responses to S-nitroso-N-acetylpenicillamine and 8-bromoguanosine 3',5'-cyclic monophosphate were similarly unchanged by iberiotoxin. In contrast, iberiotoxin attenuated the dilation resulting from moderate and severe hypoxia (arterial PO2 approximately 35 and 25 mmHg, respectively; 27 +/- 1 vs. 21 +/- 2 and 34 +/- 1 vs. 16 +/- 2% for moderate and severe hypoxia in the absence and presence of iberiotoxin, respectively). Iberiotoxin blocked responses to the K(Ca)-channel agonist NS-1619, whereas responses to the ATP-sensitive K+ agonist cromakalim were unchanged (8 +/- 1 and 15 +/- 1 vs. 1 +/- 1 and 1 +/- 1% for 10(-8) and 10(-6) M NS-1619 in the absence and presence of iberiotoxin, respectively). These data show that NO and guanosine 3',5'-cyclic monophosphate do not elicit dilation via K(Ca)-channel activation. However, activation of K(Ca) channels does contribute to hypoxic pial dilation. Finally, these data suggest that substances other than NO are involved in the contribution of K(Ca)-channel activation to hypoxic pial artery dilation.