ROLE OF CA2-DEPENDENT K-CHANNELS IN THE MEMBRANE-POTENTIAL AND CONTRACTILITY OF AORTA FROM SPONTANEOUSLY HYPERTENSIVE RATS()

1 Contractile responses to KCl and membrane potentials were determined in aortic rings from spontaneously hypertensive rats (SHR), normotens ive Wistar rats (NWR) and Wistar Kyoto rats (WKY) both in the absence and in the presence of the Ca2+-dependent K-channel blockers, apamin a nd tetraethylammonium (TEA). 2 Compared to NWR, aortic rings from WKY and SHR were less reactive and their Ca2+ uptake after stimulation wit h K+ was decreased. 3 Smooth muscle cell membrane potentials were high er in aortae from SHR and WKY than in NWR aortae, whereas SHR had high er K+ and lower Na+ intracellular activities than WKY and NWR, suggest ing overactivity of the Na+/K+ pump in the hypertensive animals. 4 Tre atment with apamin caused depolarization of WKY and SHR aortae, and in creased their contractile responses to the same level as those of the NWR. Treatment with TEA also caused depolarization of aortae from WKY and SHR, but in the SHR the depolarization induced by TEA was smaller than that produced by apamin and the contractile responses to KCl did not reach the level of those of aortae from NWR. 5 It is concluded tha t overactivity of Ca2+-dependent K-channels in aortae of WKY and SHR c ontributes to their higher membrane potentials and lower responsivenes s to vasoconstrictor stimuli. In SHR, an overactive Na+/K+ pump is als o present, and the contribution of apamin-sensitive Ca2+-dependent K-c hannels to the membrane potential and reactivity appears to be more re levant than that of TEA-sensitive channels.