Concentration-dependent, biphasic effect of prostaglandins on avian corticosteroidogenesis in vitro.

Previous work with mammalian and frog adrenocortical tissue and cells indicates that prostaglandins (PGs) can directly stimulate corticosteroidogenesis. However, work with avian adrenal preparations is absent. Therefore, the present studies with isolated chicken (Gallus gallus domesticus) and turkey (Meleagris gallopavo) adrenal steroidogenic cells were conducted to determine whether PGs can directly influence avian corticosteroidogenesis as well. Cells (1 x 10(5) cells/ml) were incubated with a wide range of concentrations of PGs in the presence of indomethacin (1 microg/ml) (to attenuate endogenous PG production) and 1-methyl-3-isobutylxanthine (0.5 mM) [to preserve cyclic AMP (cAMP)] for 2 h. Corticosterone and cAMP production were measured by highly specific radioimmunoassay. PGI2 was without effect. With the exception of PGF2alpha, which had a slight stimulation in chicken but not in turkey cells, the influence of the other PGs on corticosterone production was biphasic. For the stimulatory phase (up to a concentration of 5 x 10(-5) M), there were prostanoid structural and avian species differences in both potency and efficacy of PGs. Overall, PGs were 11 times more potent in turkey cells than in chicken cells. However, the order of potency for stimulation was similar for both chicken and turkey cells: for chicken cells the order was PGE2 > PGE1 > PGA1 > PGB2 > PGB1 > PGF2alpha and for turkey cells it was PGE2 > PGE1 > PGA1 > PGB2 = PGB1. In contrast, PG efficacy for stimulation was greater for chicken cells. In addition, the orders of efficacy were different from the orders of potency. In chicken cells, the order of efficacy was PGE2 = PGA1 > PGE1 > PGB2 > PGB1 > PGF2alpha and for turkey cells it was PGB2 = PGE2 > PGA1 > PGE1 > PGB1. Because of the greater maximal corticosterone response over basal production of chicken cells to PGs, they were used to assess the interaction of PGs with ACTH and to examine more fully the inhibitory phase of PGs. Cells were incubated with PGs in the presence of threshold (2.5 x 10(-11) M), half-maximal (1 x 10(-10) M), and maximal (1 x 10(-7) M) steroidogenic concentrations of ACTH. With the exception of PGF2alpha, the average efficacy of PGs to elevate corticosterone was increased 55% by a threshold steroidogenic concentration of ACTH. However, with higher concentrations of ACTH, this enhancement of efficacy disappeared as did the stimulatory effect of some PGs. The results suggest that the steroidogenic actions of PGs and ACTH converge on the same pool of steroidogenic enzymes leading to corticosterone. At concentrations greater than 5 x 10(-5) M, several PGs (notably PGA1, PGA2, PGB1, and PGB2) inhibited both ACTH-induced and basal corticosterone production. PGA1 and PGA2 were the most potent inhibitors. Corticosterone and cAMP production were closely associated in the biphasic action of PGs, suggesting that the effect of PGs was mediated by the changing levels of intracellular cAMP. Collectively, these data suggest that PGs may be important modulators of corticosteroidogenesis in the avian adrenal gland.

The effects of prednisone and azathioprine on circulating immunoglobulin levels and lymphocyte subpopulations in normal dogs.

This study investigates serum immunoglobulin (SIg) levels and lymphocyte subpopulations in normal dogs in response to putative immunosuppressive doses of prednisone and/or azathioprine. The objectives were to quantify SIg levels and lymphocyte subpopulations, including Thy-1+, CD4+, CD8+ and B cells, in normal dogs both before and after the administration of prednisone and/or azathioprine at 2 mg/kg, PO, each. Eighteen beagles were divided into 3 groups of 6 dogs each. Blood samples for radial immunodiffusion assay of IgG, IgM and IgA, complete blood count (CBC)and flow cytometry were collected prior to the administration of any drugs and again after 14 d of azathioprine, prednisone or azathioprine and prednisone. Peripheral blood mononuclear cells were isolated using density centrifugation and were incubated with monoclonal antibodies reacting with CD4+, CD8+, Thy-1+ and membrane immunoglobulin. Lymphocyte subsets were quantified using flow cytometry. Azathioprine-treated dogs had no significant changes in SIg levels or lymphocyte subpopulations. Prednisone-treated dogs had significant (P < 0.05) decreases in all SIg levels, all lymphocyte subpopulations and erythrocyte numbers, and had an increase in neutrophil counts. Prednisone and azathioprine-treated dogs had significant (P < 0.05) decreases in serum IgG levels and Thy-1+ and CD8+ lymphocyte subpopulations, with an increase in the CD4:CD8. These dogs also had a significant decrease in erythrocyte number and a significant increase in the monocyte count. These findings suggest that azathioprine and prednisone in combination or prednisone alone may be useful for the treatment of T cell-mediated diseases since decreased circulating T cell levels were demonstrated following treatment. The combination of drugs or azathioprine alone may not be appropriate for treatment of acute or autoantibody-mediated immune disease, because SIg levels were minimally affected by treatment.