Baroreflex control of arterial pressure under simulated micro gravity in dogs

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The purpose of the present study was to evaluate an acute modification of baroreflex control of RSNA in conscious dogs. Simultaneous recordings of carotid sinus nerve activity (CSNA) and RSNA were made during raising and lowering systemic arterial pressure by administrations of phenylephrine (150 ug iv.) and nitroprusside (300 ug iv.), respectively, in 9 conscious dogs. Relationship between mean carotid arterial pressure (Pmca) and RSNA was analyzed by fitting obtained values to a sigmoidal equation ; Pmca=A1/(1+exp(A2(X-A3))+A4 ; where X is RSNA ; A1, a range of Pmca ; A2, a gain of Pmca-RSNA response ; A3, a midpoint pressure (Pmca at the maximum gain) ; A4, a minimum value of Pmca. WI induced an acute modification of carotid sinus baroreflex control of RSNA, because A2 increased by 84%, A3 decreased by 17 mmHg, whereas A1 and A4 were constant. These data indicate that WI results in a leftward shift of the Pmca-RSNA relationship. The Pmca-CSNA relationship was analyzed similarly to study nature of carotid sinus baroreceptors during WI. The Pmca-CSNA curve did not change during control, WI and recovery periods, indicating that the modification of Pmca-RSNA relationship was not attributed to changes in a nature of carotid sinus baroreceptors. It is concluded that WI induces an acute modification of baroreflex control of RSNA at site of the supra-carotid baroreceptors.