Temperature effects on the cardiorespiratory control of American bullfrog tadpoles based on a non-invasive methodology

Abstract

Temperature effects on cardiac autonomic tonus in amphibian larval stages have never been investigated. Therefore, we evaluated the effect of different temperatures (15°C, 25°C, and 30°C) on the cardiorespiratory rates and cardiac autonomic tonus of premetamorphic bullfrog tadpoles, Lithobates catesbeianus. To this end, a non-invasive method was developed to permit measurements of electrocardiogram (ECG) and buccal movements (fB; surface electromyography of the buccal floor). For evaluation of autonomic regulation, intraperitoneal injections of Ringer's solution (control), atropine (cholinergic muscarinic antagonist), and sotalol (β-adrenergic antagonist) were performed. Ringer's injections did not affect heart rate (fH) and fBacross temperatures. Cardiorespiratory parameters were significantly augmented by temperature [fH (beats.min−1): 15°C: 24.5±1.0; 25°C: 54.5±2.0; 30°C: 75.8±2.8 and fB (movements.min−1): 15°C: 30.3±1.1; 25°C: 73.1±4.0; 30°C: 100.6±3.7]. A predominant vagal tone was observed at 15°C (32.0±3.2%) and 25°C (27.2±6.7%) relative to the adrenergic tone. At 30°C, the adrenergic tone increased relative to the lower temperature. In conclusion, the cholinergic and adrenergic tones seem to be independent of temperature for colder thermal intervals (15-25°C), while exposure to a hotter ambient temperature (30°C) seems to be followed by a significant increase in adrenergic tone and may reflect cardiovascular adjustments made to match oxygen delivery to demand. Furthermore, while excluding the use of implantable electrodes or cannulae, this study provides a suitable non-invasive method for investigating cardiorespiratory function (cardiac and respiratory rates) in water-breathing animals, like the tadpole.