USE OF POTASSIUM-DEPLETION TO ASSESS ADAPTATION OF RUMINAL BACTERIA TO IONOPHORES

When mixed ruminal bacteria from cattle fed timothy hay were suspended in a medium containing a low concentration of potassium, monensin and lasalocid catalyzed a rapid depletion of potassium from cells, The io nophore-mediated potassium depletion was concentration dependent, and it was possible to describe the relationship with saturation constants . Mixed ruminal bacteria never lost more than 50% of their potassium ( K-max 46%), and the concentrations of monensin and lasalocid needed to cause half-maximal potassium depletion (K-d) were 178 and 141 nM, res pectively, When cattle were fed 350 mg of monensin per day, the ratio of ruminal acetate to propionate decreased from 4.2 to 2.9, and the K- d of monensin was eightfold greater than the value for mixed ruminal b acteria from control animals. Monensin supplementation also caused a t wofold increase in the K-d of lasalocid. Lasalocid supplementation (35 0 mg per day) had no effect on the ruminal acetate-to-propionate ratio , but it caused a twofold increase in the K-d values of monensin and l asalocid, Increases in K-d occurred almost immediately after ionophore was added to the ration, and the K-d values returned to their prefeed ing values within 14 days of withdrawal. Ionophore supplementation had no effect on the K-max values, and approximately 50% of the populatio n was always highly ionophore resistant, Because the K-d values of eve n adapted ruminal bacteria were low (< 1.5 mu M), it appears that a la rge proportion of the ruminal ionophore is bound nonselectively to fee d particles or ionophore-resistant bacteria.