Seasonal temperature influence on vagal contol of diving bradycardia in the frog (Rana Pipiens).

Basically similar physiological adjustments occur in diving amniote vertebrates and in fish removed from water (Andersen, 1966; and additional reviews by Irving, 1939, 1964; Scholander, 1940, 1961-62, 1963, 1964; and Andersen, 1964). In these vertebrates there is a pronounced, vagally induced bradycardia (slow heart rate) which generally is achieved in a few seconds or a few minutes. Jones & Shelton (1964) have reported, however, that diving bradycardia in the frog (Rana temporaria) required 15-30 min. to become fully established and was to a great extent independent of the vagus nerve. Other amphibians (R. pipiens, R. esculenta, Xenopus muelleri, X. laevis, Bufo bufo) display a similar slowly developed, non-reflex, diving bradycardia (Shelton & Jones, 1965; Jones, 1966, 1967). This difference between amphibians and other vertebrates is puzzling. There is also additional conflicting evidence. Leivestad (i960) reported that the pattern of diving bradycardia in the toad (Bufo bufo) suggested reflex mechanisms. For the first 20 min. of a dive the heart rate alternated in quick succession between bradycardia and tachycardia, although averaging over time revealed a gradual slowing; metabolism also gradually decreased. It is our intent, therefore, to analyse diving bradycardia in the frog by taking into account three factors in an attempt to clarify the nature of the controlling mechanisms. First, amphibians do not necessarily enter an asphyxiant environment when diving; cutaneous respiration may eliminate the need for drastic and rapid physiological adjustments. This, however, depends on the second factor, temperature, which has a twofold effect; (1) metabolism increases with increasing temperature (Qio~) so that cutaneous respiration (Qio*) may be unable to meet oxygen demands, and (2) temperature directly influences vagal reflexes. Young (1959), using vagus-heart preparations of R. pipiens, reported that with decreasing temperature cardiac inhibition by the vagus was partially blocked at 14 C.; complete block occurred at io° C. When temperature was again increased, a partial response did not return until a temperature of 14-16° C. had been reached, while complete vagal inhibition of the heart was not re-established below about 18° C. Jones and Shelton (1964) conducted their experiments between 12 and 18 C, and their records are primarily at the lower temperatures; in light of