A Critical Examination of the Physical and Adrenergic Factors Affecting Blood Flow through the Gills of the Rainbow Trout

Branchial resistance decreases as cardiac output and oxygen consumption increase during exercise in trout (Stevens & Randall, 1967a, b; Stevens, 1968a, b) so blood flow rises to a much greater extent than blood pressure. Similar changes also probably occur during real or CO-induced anaemia (Cameron & Davis, 1970; Holeton, 1971) but not during hypoxia (Holeton & Randall, 1967a, b). A large number of studies on excised and/or perfused teleostean gill preparations have demonstrated that catecholamines cause branchial vasodilation (Krawkow, 1913; Keys & Bateman, 1932; Ostlund & Fange, 1962; Maetz & Rankin, 1969; Kirschner, 1969; Reite, 1969; Rankin & Maetz, 1971; Belaud, Peyraud-Waitzenegger & Peyraud, 1971; Randall, Baumgarten & Maylusz, 1972), accompanied by the opening of respiratory shunts in the secondary lamellae (Steen & Kruysse, 1964; Richards & Fromm, 1969). Investigations on intact animals have implied that exogenous adrenaline has a similar inhibitory effect on the gills in vivo (Mott, 1951; Randall & Stevens, 1967; Reite, 1969; Maetz & Rankin, 1969; Johansen, 1972). It is therefore thought that the observed rise in circulatory catecholamine levels during swimming activity (Nakano & Tomlinson, 1967) mediates the branchial adaptations to exercise in Salmo gairdneri (Randall, 1970; Johansen, 1972). Yet, for reasons long recognized in mammalian physiology, the aforementioned work is subject to severe limitations. Calculations of branchial resistance shifts based on blood pressure and flow changes measured in vivo, such as those performed by Stevens (1968 a, b) and Johansen (1972) do not necessarily indicate alterations in vascular tone. Resistance can change quite significantly in response to purely physical influences when blood pressures and flows vary, without any alterations in tone of local, humoral, or autonomic origin (Whittaker & Winton, 1933; Green et al. 1944; Burton, 1951; Kuida, 1965). These factors are the distensibility of resistance vessels and the anomalous viscosity characteristics of blood, the former being considered predominant (Kuida, 1965). The peculiar situation of the gills as a resistance between two arterial pressures which are to a certain extent independently controlled may make passive changes in blood-vessel radius or number of particular importance in their function. Because of this potential distensibility, the aforementioned studies on ^olated/perfused preparations must be considered quantitatively applicable only to