Active sulfate absorption in rabbit ileum: Dependence on sodium and chloride and effects of agents that alter chloride transport

Summary Unidirectional fluxes of³⁵SO₄ across and into rabbit ileal epithelium were measured under short-circuit conditions, mostly at a medium SO₄ concentration of 2.4mm. Unidirectional mucosa (m)-to-serosa (s) ands-to-m fluxes (J_ms,J_sm) were 0.456 and 0.067 moles hr^–1 cm^–2, respectively.J_ms was 2.7 times higher in distal ileum than in mid-jejunum. Ouabain abolished net SO₄ transport (J_net) by reducingJ_ms. Epinephrine, a stimulus of Cl absorption, had no effect on SO₄ fluxes. Theophylline, a stimulus of Cl secretion, reducedJ_ms without affectingJ_sm, causing a 33% reduction inJ_net. Other secretory stimuli (8-Br-cAMP, heat-stable enterotoxin, Ca-ionophore A23187) had similar effects. Replacement of all Cl with gluconate markedly reducedJ_net through both a decrease inJ_ms and an increase inJ_sm. The anion-exchange inhibitor, 4-acetoamido-4-isothiocyano-2,2-sulfonic acid stilbene (SITS), when added to the serosal side, reducedJ_ms by 94%, nearly abolishingJ_net. SITS also decreasedJ_sm by 75%. Mucosal SITS (50 m) was ineffective. 4,4-diisothiocyano-2,2-sulfonic acid stilbene (DIDS) had effects similar to SITS but was less potent. Measurements of initial rates of epithelial uptake from the luminal side (J_me) revealed the following: (1)J_me is a saturable function of medium concentration with aV_max of 0.94 moles hr^–1 cm^–2 and aK_1/2 of 1.3mm; (2) replacing all Na with choline abolishedJ_me; (3) replacing all Cl with gluconate increasedJ_me by 40%; (4) serosal SITS had no effect onJ_me; and (5) stimuli of Cl secretion had no effect onJ_me or increased it slightly. Determination of cell SO₄ with³⁵SO₄ indicated that, at steady-state, the average mucosal concentration is 1.1 mmoles per liter cell water, less than half the medium concentration. Cell SO₄ was increased to 3.0mm by adding SITS to the serosal side. Despite net transport rates greater than 1.4 Eq hr^–1 cm^–2, neither addition of SO₄ to the SO₄-free medium nor addition of SITS to SO₄-containing medium altered short-circuit current. The results suggest that (1) ileal SO₄ absorption consists of Na-coupled influx (symport) across the brush border and Cl-coupled efflux (antiport) across the basolateral membrane; (2) the overall process is electrically neutral; (3) the medium-to-cell Cl concentration difference may provide part of the driving force for net SO₄ absorption; and (4) since agents affecting Cl fluxes (both absorptive and secretory) have little effect on SO₄ fluxes, the mechanisms for their transcellular transports are under separate regulation.