Competitive antagonism between calcium and aminoglycoside antibiotics on guinea-pig intestinal smooth muscle

Streptomycin and related aminoglycoside antibiotics and magnesium ions antagonize in a competitive manner the contractile response elicited by calcium ion on the potassium-depolarized guinea-pig ileum. The inhibitory potency decreases in the following way: gentamicin greater than magnesium greater than neomycin greater than streptomycin=streptidine greater than kanamycin. Streptomycin was also found to reduce the uptake and to increase the efflux of 45Ca. To explain these effects two alternative mechanisms are proposed: 1) a competition of the antibiotics with calcium ions for binding sites of a transport system which carries the activator ion to the myoplasm, and 2) an interaction of the antibiotics with membrane sites that regulate calcium permeability with a consequent reduction of calcium passage to the cell.