INTRODUCTION. Lithium salts are prescribed as mood stabilizers to treat mental disorders. It has been widely reported that lithium therapy negatively affects collecting duct principal cells resulting in reduced urinary concentrating ability. Less is known about the effects of lithium salts on the intercalated cells and acid-base transport in the collecting duct. Previous studies have shown that activation of G_q-coupled vasopressin 1a receptors (V1aR) in type A intercalated cells stimulates luminal H⁺ secretion resulting in urinary acidification, while lithium therapy increases baseline urine pH. We hypothesize that lithium administration markedly reduces V1aR-dependent Ca²⁺ signaling in intercalated cells impairing the regulation of acid-base transport in the collecting duct.

METHODS. We combined immunofluorescent imaging in freshly isolated collecting duct segments with metabolic cage studies in C57BL/6NJ (Jackson Labotatory) mice receiving a regular chow (Teklad 2918) or a diet containing 0.3% of lithium carbonate (added to the base Teklad 2918 chow) to assess the effects of lithium on V1aR-mediated Ca²⁺ signal, H⁺ transport by intercalated cells, as well as urinary pH, ammonia and acid excretion.

RESULTS. We found that application of arginine-vasopressin (AVP, 1 nM) elicits a transient calcium response in the aquaporin-2 (AQP2) negative cells from the collecting duct segments isolated from the mice on a regular chow. Calcium in response to AVP was mobilized from the intracellular stores as the AVP-dependent calcium response was preserved in 0 Ca²⁺ extracellular solution and ablated upon pretreatment of collecting duct segments with SERCA pump inhibitor, thapsigargin, 500 nM. The amplitude of AVP-induced calcium transient was markedly reduced in the AQP2-negative cells isolated from lithium-treated mice when compared to controls (39±5 vs 163±40 nM, respectively). The reduction of AVP-mediated Ca²⁺ signaling in the AQP2-negative collecting ducts cells was paralleled with significant changes in proton extrusion rates and acid excretion with urine.

CONCLUSIONS. Our findings reveal that AVP-stimulated intracellular Ca²⁺ signaling in intercalated cells is an important determinant of acid transport in the collecting duct and urinary acid excretion at the systemic level. Lithium treatment significantly impairs AVP-dependent intracellular Ca²⁺ signaling in the collecting duct intercalated cells, significantly altering H⁺ handling by intercalated cells and acid excretion with urine.

ETHICAL STANDARDS. All animal procedures were approved by the Institutional Animal Care and Use Committee of the Medical College of Georgia at Augusta University (AUP 2017-0844) and were conducted in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals.