Temperature-dependent development of cardiac activity in unrestrained larvae of the minnow Phoxinus phoxinus

The minnow (Phoxinus phoxinus) was raised up to the stage of swim bladder inflation at temperatures between 10 degreesC and 25 degreesC, and the time of development significantly decreased at higher temperatures. Accordingly, initiation of cardiac activity was observed at day 2 in 25 degreesC animals and at day 4 in 12.5 degreesC animals. Only a minor increase in body mass was observed during the incubation period, and, at the end of the incubation period, animals raised at 25 degreesC did not have a significantly lower body mass compared with animals raised at 15 degreesC. Metabolic activity, determined as the rate of oxygen consumption of a larva, increased from 3.3 to 19.5 nmol/h during development at 15 degreesC and from 5.6 to 47.6 nmol/h during development at 25 degreesC. Heart rate showed a clear correlation to developmental stage as well as to developmental temperature, but at the onset of cardiac activity, diastolic ventricular volume and also stroke volume were higher at the lower temperatures. Furthermore, stroke volume increased with development, except for the group incubated at 12.5 degreesC, in which stroke volume decreased with development. Initial cardiac output showed no correlation to incubation temperature. Although metabolic activity increased severalfold during development from egg to the stage of swim bladder inflation at 5 degreesC and at 25 degreesC, weight-specific cardiac output increased only by similar to 40% with proceeding development. At 12.5 degreesC, cardiac output remained almost constant until opening of the swim bladder. The data support the notion that oxygen transport is not the major function of the circulatory system at this stage of development. The changes in heart rate with temperature appear to be due to the intrinsic properties of the pacemaker; there was no indication for a regulated response.