Stress-associated concentrations of plasma cortisol cannot account for reduced lymphocyte function and changes in serum enzymes in lambs exposed to restraint and isolation stress.

Lambs were used to evaluate the effect of stress-associated plasma concentrations of cortisol on measures of immune function. Lambs given restraint and isolation stress (RIS) were isolated from visual and tactile contact with other lambs for 6 h on three consecutive days. Additional lambs were infused with cortisol (CORT) to increase plasma concentrations similar to those in lambs subjected to RIS. Control lambs (CON) were undisturbed. On the 1st and 3rd d of the experiment, plasma was obtained hourly during treatment for quantification of cortisol and ACTH. Blood mononuclear cells were obtained 24 h before the 1st d of treatment and daily at the conclusion of treatment for responses to phytohemagglutinin (PHA), concanavalin A (Con A), and pokeweed mitogen (PWM); for production of interleukin 2 (IL2); and for identification of cells expressing major histocompatibility complex class II antigens (MHCII). Combined fitted profiles of cortisol in RIS and CORT lambs differed (P < .005) from those in CON on both days evaluated. Fitted profiles of cortisol were similar between RIS and CORT lambs on the 1st d of treatment but differed (P < .005) on the 3rd d. Combined fitted profiles of ACTH from CON and CORT lambs differed from those of RIS lambs on both days of treatment (P < .005). Lymphocyte responses to PHA (P < .01), Con A (P < .01), and PWM (P < .07) were less in RIS than in CON but did not differ between CON and CORT. Treatment did not affect IL2 or MHCII. Increased plasma cortisol measured during RIS cannot alone account for reduced lymphocyte proliferative responses.