Circadian changes in physiological responses and blood ionized sodium and potassium concentrations under thermal exposure in Tharparkar and Karan Fries heifers

Abstract

To explore the circadian changes in physiological responses and blood ionized sodium, potassium concentrations under thermal exposure in Tharparkar (TP) and Karan Fries (KF) heifers, 12 animals, six each from two breeds were taken and kept first in control thermoneutral (21–268C and 50–60% RH) and in heat exposed (388C and 50–60% RH) conditions thereafter. Throughout a 24-hour period physiological responses (rectal temperature, respiration rate and pulse rate) and blood samples were taken at 4-hour intervals, then blood samples were analyzed for sodium (Na+) and potassium (K+) levels. Significant circadian variations in physiological responses and blood Na+ and K+ within 24 hours have been observed. Significant (P 5 0.01) variation in physiological responses and blood Na+, K+ concentrations were observed in both Tharparkar and Karan Fries heifers under control and heat exposed conditions. Values of physiological responses were lower in the morning and showed an increasing trend and reached the highest value in the afternoon. Blood Na+ and K+ concentrations decreased from 11.00 hours to 19.00 hours due to heat exposure. The decreases in blood electrolyte levels were greater in Karan Fries in comparison to Tharparkar heifers that expressed less heat tolerance capacity in crossbred than indigenous cattle. This finding suggests that it is important to standardize the time of sample collection in comparative studies, in which more discrete differences may be identified in physiological responses and blood electrolyte concentrations