Cerebrospinal fluid [Ca2+] and rectal temperature response during exercise in dogs

Rectal temperature (Tre) was measured in five dogs during 1 h of rest and 1 h of moderate treadmill exercise (1.2 m/s up a 12 degree slope), with no infusion and with continuous infusion (40 mul/min) of normocalcic (1.3 mM Ca2+, 151.5 mM Na+) and hypercalcic (2.6 mM Ca2+, 149.6 mM Na+) concentrations of artificial cerebrospinal fluids into the left lateral cerebral ventricle. There was no effect of the normo- or hypercalcic infusions on resting Tre. In comparison with the postexercise Tre level of 39.9 degrees C (deltaTre= +1.4 degrees C) with no infusion, Tre rose to 40.0 degrees C (delta Tre = +1.4 degrees C) with normocalcic infusion (NS); there was a significantly smaller (P less than 0.01) rise in Tre to 39.2 degrees C (delta Tre = +0.8 degrees C) with hypercalcic infusion. The smaller rise in Tre in response to excess Ca2+ in cerebrospinal fluid was not due to plasma fluid or electrolyte shifts. During exercise the mean body weight loss was greater with hypercalcic infusion (-2.7%) compared with normocalcic (-1.0%) and no infusion (-1.4%) values. The results suggest that Ca2+ ions affect the sensitivity of the central thermoreceptor to thermal stimuli and cause an increased heat loss during physical exercise in dogs.