α-Adrenergic regulation of systemic peripheral resistance and blood flow distribution in the turtle Trachemys scripta during anoxic submergence at 5°C and 21°C

Anoxic exposure in the anoxia-tolerant freshwater turtle is attended by substantial decreases in heart rate and blood flows, but systemic blood pressure (P_sys) only decreases marginally due to an increase in systemic peripheral resistance (R_sys). Here,we investigate the role of the α-adrenergic system in modulating R_sys during anoxia at 5°C and 21°C in the turtle Trachemys scripta, and also describe how anoxia affects relative systemic blood flow distribution(%Q̇_sys) and absolute tissue blood flows. Turtles were instrumented with an arterial cannula for measurement of P_sys and ultrasonic flow probes on major systemic blood vessels for determination of systemic cardiac output(Q̇_sys). α-Adrenergic tone was assessed from vascular injections of α-adrenergic agonists and antagonists (phenylephrine and phentolamine, respectively) during normoxia and following either 6 h (21°C) or 12 days (5°C) of anoxic submergence. Coloured microspheres, injected through a left atrial cannula during normoxia and anoxia, as well as after α-adrenergic stimulation and blockade during anoxia at both temperatures, were used to determine relative and absolute tissue blood flows.

Anoxia was associated with an increased R_sys and functional α-adrenergic vasoactivity at both acclimation temperatures. However, while anoxia at 21°C was associated with a high systemicα-adrenergic tone, the progressive increase of R_sysat 5°C was not mediated by α-adrenergic control. A redistribution of blood flow away from ancillary vascular beds towards more vital circulations occurred with anoxia at both acclimation temperatures.%Q̇_sys and absolute blood flow were reduced to the digestive and urogenital tissues (approximately 2- to 15-fold), while %Q̇_sys and absolute blood flows to the heart and brain were maintained at normoxic levels. The importance of liver and muscle glycogen stores in fueling anaerobic metabolism were indicated by increases in%Q̇_sys to the muscle at 21°C (1.3-fold) and liver at 5°C (1.7-fold). As well, the crucial importance of the turtle shell as a buffer reserve during anoxic submergence was indicated by 40-50% of Q̇_sys being directed towards the shell during anoxia at both 5°C and 21°C. α-Adrenergic stimulation and blockade during anoxia caused few changes in%Q̇_sys and absolute tissue blood flow. However, there was evidence of α-adrenergic vasoactivity contributing to blood flow regulation to the liver and shell during anoxic submergence at 5°C.