"Cascade" principle of the control of non-shivering thermogenesis by intravenously infused noradrenaline.

177 A hypothesis is presented according to which noradre­ naline (NA) infused into the blood reaches the biophase of thermogenic cells through three "cascade" steps, namely the blood volume, extravascular space and biophase. The influx rate of the NA entering the system is given by a forcing function and multiplied by cardiac output. Distribution of the NA at each step is identically described by three differential equations. Since in cold-acclimated rats skeletal muscles parti­ cipate to a great extent in total NST and since adrenergic terminals in muscles end in the wall of their blood vessels, a long distance between adrenergic terminals and effectors should be also anticipated during control of non-shivering thermogenesis (NST), particularly one located in muscles. The control of NST is realized, therefore, by interaction of different mechanisms, distributing NA during the trans­ mission. This interaction determines a resulting concentra­ tion of NA in biophase, which stimulates metabolic responses of the cells. To clarify the control of NST by sympathetic nervous endings, a system distributing i.v. infused NA in anesthetized cold acclimated rats at thermoneutral zone was analysed.