Table 1.

Effect of pH and anion gradients on l-lactate uptake

	l-Lactate uptake (pmol (mg protein)−1 (5s)−1)
Conditions	Mannitol loaded	l-Lactate loaded
pHo = pH1 = 7.5	38 ± 3	87 ± 4*
pHo = 5.5; pH1 = 7.5	134 = 14	296 ± 25*
pHo = pH1 = 5.5	53 ± 4	241 ± 5*
pHo = 7.5; pH1 = 5.5	24 ± 5	67 ±4*

LMV were isolated as described in Methods. Vesicles were preloaded with either 300 mm mannitol or with 100 mm mannitol and 100 mm sodium l-lactate, 0.1 mm MgSo₄, and either 20 mm Mes-Tris pH 5.5 or 20 mm Hepes-Tris pH 7.5. Membrane vesicles (100 μg of protein per assay) were incubated in the standard uptake buffer containing 100 mm mannitol, 100 mm sodium gluconate, 0.1 mm MgSO₄, 0.1 mml[U-¹⁴C]lactate and either 20 mm Mes-Tris PH 5.5 or 20 mm Hepes-Tris pH 7.5. Uptake was measured at 37°C as described in Methods. The reaction was stopped after 5 s by the addition of 1 ml ice-cold stop buffer (100 mm mannitol, 100 mm sodium gluconate, 20 mm Hepes-Tris pH 7.5, 0.1 mm MgSO₄ and 0.1 mm sodium l-lactate) and samplews were filtered under vacuum. Values are presented as means ±s.e.m. for three experiments.

^*Statistically significant (P < 0.05) compared with the matched control (mannitol loaded).