alpha-Adrenergic regulation of systemic peripheral resistance and blood flow distribution in the turtle Trachemys scripta during anoxic submergence at 5 degrees C and 21 degrees 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 alpha-adrenergic system in modulating R-sys during anoxia at 5degreesC and 21degreesC in the turtle Trachemys scripta, and also describe how anoxia affects relative systemic blood flow distribution (%Q(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 (Q(sys)). alpha-Adrenergic tone was assessed from vascular injections of alpha-adrenergic agonists; and antagonists (phenylephrine and phentolamine, respectively) during normoxia and following either 6 h (21degreesC) or 12 days (5degreesC) of anoxic submergence. Coloured microspheres, injected through a left atrial cannula during normoxia and anoxia, as well as after a-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 a-adrenergic vasoactivity at both acclimation temperatures. However, while anoxia at 21degreesC was associated with a high systemic a-adrenergic tone, the progressive increase of R-sys at 5degreesC was not mediated by alpha-adrenergic control. A redistribution of blood flow away from ancillary vascular beds towards more vital circulations occurred with anoxia at both acclimation temperatures. %Q(sys) and absolute blood flow were reduced to the digestive and urogenital tissues (approximately 2- to 15-fold), while %Q(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 %Q(sys) to the muscle at 21degreesC (1.3-fold) and liver at 5degreesC (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 Q(sys) being directed towards the shell during anoxia at both 5degreesC and 21degreesC. alpha-Adrenergic stimulation,, and blockade during anoxia caused few changes in %Q(sys) and absolute tissue blood flow. However, there was evidence of alpha-adrenergic vasoactivity contributing to blood flow regulation to the liver and shell during anoxic submergence at 5degreesC.