Camp. Biochem. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Physiol Vol. 93C, No. I, pp. 105-l IO, 1989 Printed in Great Britain 0306-4492/89 $3.00 + 0.00 0 1989 Pergamon Press plc BETA-ADRENERGIC SENSITIVITY IN JUVENILE AMERICAN COOTS (WLICA AMERICANA): EVIDENCE OF NONSHIVERING THERMOGENESIS? GLENN C. SUTTER and ROBERTA. MACARTHUR* Department of Zoology, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 zyxwvutsrqponmlk (Received 7 hdy 1988) Abstract-l, Ontogenetic changes in the metabolic responsiveness of American coots (Fulica americana) to /I-adrenergic stimulation were documented for hatchling through fledgling stages. 2. In coots >40days of age, intramuscular injection of the b-agonist, isoproterenol, raised thermo- neutral heat production by 3&50%; no response was evident in younger birds. 3. The metabolic response of fledglings to isoproterenol was dose-dependent, and was effectively blocked by concurrent treatment with the /?-antagonist, propranolol. 4. Treatment only with propranolol induced a slight metabolic decline in fledglings exposed to mild cold. 5. Though coots appeared to lack thermogenic brown fat, the B-adrenergic sensitivity of older birds implies the extstence of nonshivermg thermogenesis in thts species. INTRODUCTION The contribution of nonshivering thermogenesis (NST) to avian temperature regulation has generated much controversy in recent years (see Hissa et nl., 1975; Barre et al., 1985). Whereas mammalian NST arises mainly in response to /I-adrenergic stimulation of brown adipose tissue (BAT), this thermogenic fat is apparently lacking in most birds (Johnston, 1971). Yet several avian studies have demonstrated thermo- genie responses to exogenous catecholamines (Hissa et al., 1975; Barre and Rouanet, 1983), and recent investigations have revealed depots of multilocular adipose tissue resembling BAT in black-capped chickadees, Parus atricapillus, and ruffed grouse, Bonasa umbellus (Oliphant, 1983) as well as in cold- acclimated muscovy ducklings, Anas barbariue (Barre et al., 1986a). Selection for avian NST should be particularly strong in aquatic species, since shivering in cold water greatly amplifies convective heat loss to the surround- ing medium (Boutelier et al., 1977). The need for aquatic NST would appear most critical in precocial aquatic chicks-especially those raised on high- latitude breeding grounds where neonates < 50 g may encounter cool water within days of hatching (Nice, 1962; Koskimies and Lahti, 1964). Yet surprisingly little is known of the capacity for thermoregulatory NST in these amphibious chicks. Recent studies of NST in aquatic birds have focused instead on month- old muscovy ducklings which may exceed 0.4 kg body weight (Barre et al., 1985. 1986a, 1987). The aim of this study was to examine the possible occurrence and ontogenetic development of NST in a small-bodied aquatic bird, the American coot (Fulica americana, 0. Gruiformes). Despite body weights of only 19-20 g, neonate coots may enter water as soon as they are dry, and young < 5 days old *Author to whom correspondence should be addressed. often remain in water for extended periods (Gullion, 1954). Moreover, F. americana breeds as far north as 60” latitude, and is known to overwinter at latitudes as high as 50” (Robbins et al., 1983; Godfrey, 1986). Hence this bird is an excellent model for the investi- gation of avian NST. Though the ecology of F. americana has been extensively studied (see Gullion, 1954; Alisauskas and Ankney, 1985; Desrochers and Ankney, 1986), little physiological data is available for this or any other Gruiforme (Brent et al., 1984, 1985). Our specific objective was to investigate p-adren- ergic sensitivity of F. americana during different stages of development. Metabolic rate (MR) and cloaca1 temperature (T,,) responses to the B-agonist, isoproterenol were documented in birds ranging in age from 0-l day to 60+ days posthatch. To mini- mize responses to endogenous catecholamines or shivering thermogenesis, these experiments were per- formed at thermoneutral temperatures. Responses of fledgling coots (60+ days) acclimated to room tem- perature were compared with those of cold-accli- mated birds of similar age. Attempts were also made to abolish the thermogenic response to isoproterenol using the /I-adrenergic blocker, propranolol. As a further test for the possible involvement of /I-adren- ergic pathways in the metabolic response to cold, propranolol was administered alone to fledgling coots exposed to mild cold stress. Carcasses of hatchling and fledgling coots were examined by gross dissection for the possible occurrence of BAT. MATERIALS AND METHODS Animals A total of 48 hand-reared coots was used in this study. Eggs were collected from marshes near Winnipeg and Neepawa, Manitoba during June, 1985,1986. The eggs were incubated in a commercial incubator and the hatchlings reared on a diet of mealworms, chopped earthworms, moistened dogfood, poultry crumbles and turkey starter 105