Variable open-loop gain in the control of thermogenesis in cold-exposed rabbits

Abstract

A multiplicative interaction was previously demonstrated between mean skin temperature (.hivin.Tsk) and hypothalamic temperature (Thy) in the control of thermogenesis in cold exposed rabbits. Such a model predicts that open-loop gain will not be constant, but will vary according to the direction and degree of Thy displacements and to the prevailing level of .hivin.Tsk when Thy is altered. Open-loop gain was calculated from steady-state displacements in body temperature induced by sustained step displacements in Thy. Hypothalamic temperature of rabbits was clamped at various levels ranging from 2.degree. C above to 3.degree. C below normal Thy and the resulting changes in rectal temperature attained at steady state were recorded. Experiments were conducted at ambient temperatures (Ta) of 2.degree. and 15.degree. C. Open-loop gain in response to hypothalamic heating was always greater than that obtained in response to equivalent levels of hypothalamic cooling. At lower levels of .hivin.Tsk (Ta = 2.degree. C) open-loop gain was greater for all levels of Thy displacement when compared to that measured at higher levels of .hivin.Tsk (Ta = 15.degree. C).