Monensin, a monovalent ion-selective ionophore, facilitates the transmembrane exchange of principally sodium ions for protons. The outer surface of the ionophore-ion complex is composed largely of nonpolar hydrocarbon, which imparts a high solubility to the complexes in nonpolar solvents. In biological systems, these complexes are freely soluble in the lipid components of membranes and, presumably, diffuse or shuttle through the membranes from one aqueous membrane interface to the other. The net effect for monensin is a trans-membrane exchange of sodium ions for protons. However, the interaction of an ionophore with biological membranes, and its ionophoric expression, is highly dependent on the biochemical configuration of the membrane itself. One apparent consequence of this exchange is the neutralization of acidic intracellular compartments such as the trans Golgi apparatus cisternae and associated elements, lysosomes, and certain endosomes. This is accompanied by a disruption of trans Golgi apparatus cisternae and of lysosome and acidic endosome function. At the same time, Golgi apparatus cisternae appear to swell, presumably due to osmotic uptake of water resulting from the inward movement of ions. Monensin effects on Golgi apparatus are observed in cells from a wide range of plant and animal species. The action of monensin is most often exerted on the trans half of the stacked cisternae, often near the point of exit of secretory vesicles at the trans face of the stacked cisternae, or, especially at low monensin concentrations or short exposure times, near the middle of the stacked cisternae. The effects of monensin are quite rapid in both animal and plant cells; i.e., changes in Golgi apparatus may be observed after only 2-5 min of exposure. It is implicit in these observations that the uptake of osmotically active cations is accompanied by a concomitant efflux of H+ and that a net influx of protons would be required to sustain the ionic exchange long enough to account for the swelling of cisternae observed in electron micrographs. In the Golgi apparatus, late processing events such as terminal glycosylation and proteolytic cleavages are most susceptible to inhibition by monensin. Yet, many incompletely processed molecules may still be secreted via yet poorly understood mechanisms that appear to bypass the Golgi apparatus. In endocytosis, monensin does not prevent internalization. However, intracellular degradation of internalized ligands may be prevented.(ABSTRACT TRUNCATED AT 400 WORDS)

Twenty-four lactating cows were assigned randomly to three treatments to evaluate responses to large differences of dietary sodium and chloride. Treatments were corn-cottonseed meal-corn silage based complete rations with either: 1) .23% sodium chloride (control), 2) control plus 2.28% calcium chloride, or 3) control plus 1.70% sodium bicarbonate. Treatment effects were significant for urine pH (7.96, 5.41, 8.18), blood pH (7.50, 7.39, 7.49), partial pressure of oxygen (91.2, 99.4, 86.3 mm Hg), partial pressure of carbon dioxide (34.60, 30.57, 32.98 mm Hg), bicarbonate (26.20, 18.06, 24.64 meq/liter), total carbon dioxide (27.51, 19.18, 25.88 mM/liter), base excess (4.50, -4.31, 3.13 meq/liter), plasma chloride (93.4, 102.8, 95.7 meq/liter), serum potassium (3.26, 4.24, 4.14 meq/liter), and inorganic phosphorus (7.11, 5.61, 6.80 mg/100 ml). Blood glucose (45.1, 43.0, 55.5 mg/dl) and blood urea nitrogen (11.8, 8.7, 11.9 mg/dl) exhibited treatment effects. Respiration rates, 84.8, 61.8, 89.9 per min, and body temperatures, 39.7, 39.0, and 40.0 degrees C were significantly different. Lower intake of the high chloride diet and higher intake of the bicarbonate diet were probably responsible for some of the effects. Dietary electrolytes should receive attention in formulation because acid-base status of the animal is determined, in part, by ionic concentration and balance of the diet.

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