It is likely that these two forms have the same affinity for the active site of the drug. The two dose-response curves are parallel. Sodium antibioticus has a pKa of 3. Renal elimination of the drug is solely via renal tubular secretion. Tubular secretion is a significant mode of renal elimination.
Glomerular filtration is a significant mode of renal elimination.
One administers a drug to an animal and finds upon measuring the total drug concentration in the plasma that it appears if the drug has diluted into a volume 5 times larger than the body water. Include drug concentration in local environment and blood as well as amount of drug in the original dose form. Speculate on the potential evolutionary bases for these differences.