Drug Movement between Bovine Milk and Plasma as Affected by Milk pH

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Abstract
Salicylic acid, p-aminohippuric acid, sulfacetamide, sulfamerazine, sulfamethazine, sulfathiozole, sulfa-pyridine, sulfanilamide and phenol were injected into lactating cows via the jugular vein using a wide range of inital loading doses of the particular drug. This was followed by maintenance doses designed to hold the plasma concentration constant over a 6 hr. period. The right front quarter of each cow was infused with sterile bicarbonate buffer solution in order to establish a pH value of 8.0 in that quarter. Blood, milk from the normal quarters, and "milk" from the bicarbonate treated quarters were sampled simultaneously at hourly intervals. Milk pH values were measured and drug concentrations were determined. The weak organic bases (urea, antipyrine, creatinine, aminopyrine, quinine, and ephedrine) were studied in like manner. Theoretical milk to plasma ratios which would be expected if the drug followed pH partition concepts were calculated for each drug. Experimentally determined milk to plasma ultrafiltrate ratios agreed well with theoretically calculated ratios in both the normal and bicarbonate treated quarters. These compounds seemed to follow pH partition concepts even when the fluid in the mammary gland approached a pH as high as 8.0 The passage of these compounds into bovine milk can be explained by passive diffusion.

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