Contractile responses of the basilar artery isolated from rabbits exposed to 8-day head-down tilt.

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In the present study, the effects of head-down tilt (HDT) on contractile responses of basilar and popliteal arteries were investigated in vitro. The arteries were isolated from rabbits exposed to 8 d of 45 degrees HDT. Isometric tension and intracellular calcium were measured in an organ bath perfused with physiological salt solution. In the HDT rabbits, contractile response to norepinephrine (NE) was attenuated in the basilar artery, but not in the popliteal artery compared with control rabbits. HDT did not change the responses to either KCl or 5-hydroxytryptamine in both arteries. In the response of basilar artery to NE, the difference in both [Ca(2+)](i) transient and Ca sensitivity between control and HDT rabbits were statistically not significant. The response of the basilar artery to phenylephrine, a selective alpha(1)-adrenergic agonist, was also attenuated by HDT. Treatment with propranolol, a beta-adrenergic blockade, did not affect the response to NE in the basilar arteries isolated from control rabbits, but it significantly enhanced the response in the basilar arteries from HDT rabbits. These results suggest that exposure to 8-d HDT decreases a contractile response of the basilar artery to NE in rabbits. The reduction of NE-induced contraction is probably attributable to both decreased Ca transient and decreased Ca sensitivity, and decreased alpha(1)-adrenoceptor activity and increased beta-adrenoceptor activity seem to be involved in the mechanism.