Respiratory effects of pregnancy and progesterone in Jersey cows.

Unanesthetized Jersey cows were studied during both pregnant (5-9 months) nonlactating states, and nonpregnant lactating states; and also following treatment with progesterone (Pr). The pH, PCO2 and PO2 of aortic blood, VE and f were measured and the mixed expired gas was analyzed. The following significant changes from the nonpregnant state occurred during pregnancy: PaCO2 = -3.2 mm Hg, pHa = +0.02 unit, VT = -0.44 L, f = +7 breaths/min, and VE/VCO2 = +9.7. Concomitant with the respiratory studies, serum Pr levels were determined by radioimmunoassay (RIA) in 11 nonpregnant and 5 pregnant cows, and in 6 nonpregnant, lactating cows prior to and on days 3, 5 and 10 of treatment with Pr (500 mg, i.m., twice daily). Minute ventilation (VE, L X min-1 X kg-1, BTPS) was positively correlated (r = +0.59) and PaCO2 was negatively correlated (r = -0.64) with endogenous serum Pr levels of non-pregnant and pregnant cows. However, exogenous Pr did not significantly alter these parameters or pHa, despite mean serum levels nearly twice (23.6 +/- 10.2 ng/ml) those observed in pregnant cows (12.7 +/- 3.7 ng/ml). The increased ventilation during pregnancy in Jersey cows, shown in this study, does not appear to be related to Pr as exogenous Pr failed to induce hyperventilation. The correlation of increased ventilation with endogenous Pr levels therefore suggests that the mode of in vivo Pr release, or different compound, simultaneously released, could be the stimulus.