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, 25 and 30 degrees C) on the cardiorespiratory rates and cardiac autonomic tonus of premetamorphic tadpoles of the bullfrog, Lithobates catesbeianus. To this end, a non-invasive method was developed to permit measurements of electrocardiogram (ECG) and buccal movements (f(B); surface electromyography of the buccal floor). For evaluation of autonomic regulation, intraperitoneal injections of Ringer solution (control), atropine (cholinergic muscarinic antagonist) and sotalol (beta-adrenergic antagonist) were performed. Ringer solution injections did not affect heart rate (fH) or fB across temperatures. Cardiorespiratory parameters were significantly augmented by temperature (fH: 24.5 +/- 1.0, 54.5 +/- 2.0 and 75.8 +/- 2.8 beats min(-1) at 15, 25 and 30 degrees C, respectively; fB: 30.3 +/- 1.1, 73.1 +/- 4.0 and 100.6 +/- 3.7 movements min(-1) at 15, 25 and 30 degrees C, respectively). A predominant vagal tone was observed at 15 degrees C (32.0 +/- 3.2%) and 25 degrees C (27.2 +/- 6.7%) relative to the adrenergic tone. At 30 degrees 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 degrees C), while exposure to a hotter ambient temperature (30 degrees 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 such as the tadpole.