Background: Ca²⁺-activated Cl^– channels encoded by the TMEM16A gene are highly expressed in arterial smooth muscle cells (aSMCs) and in contractile cerebral pericytes. In these cells, TMEM16A activation induces membrane depolarisation and vessel constriction. The FDA-approved drug niclosamide has been shown to modulate the TMEM16A channel (1-3) and thus may constitute a new therapeutic agent for diseases of altered vessel tone including stroke, hypertension and vascular dementia (4,5). Here, we investigate the effects of niclosamide, at concentrations comparable to those observed in plasma of patients treated with niclosamide, on (i) TMEM16A currents and (ii) the tone of isolated arteries and capillaries.

 Methods: Whole-cell patch-clamp recordings of native and cloned TMEM16A currents were used to investigate how niclosamide modulates the TMEM16A channel. Isometric tension recordings of isolated rat aortae, pulmonary and mesenteric arteries, as well as Differential Interference Contrast (DIC) imagining of acute rat brain slices, were used to investigate how channel regulation by niclosamide affects vessel tone.

Results: Clinically-relevant niclosamide concentrations (≤1 μM) were examined on heterologous TMEM16A currents in HEK293T cells under various membrane potential (V_m) and intracellular free Ca²⁺ ([Ca²⁺]_i) conditions. At 0.3 µM [Ca²⁺]_i, niclosamide induced a biphasic effect by inhibiting at positive V_m  (by 97±1.6% (n=8) at +100 mV)  and potentiating at negative V_m (by 20±4.3 (n=10) fold at -100 mV) . These effects diminished as the [Ca²⁺]_i was raised to levels that cause half-maximal or maximal TMEM16A activation.

Niclosamide (1 µM) reduced responses to the phenylephrine, contractile G_qPCR agonist, in isolated rat aortae (by 81.0±3.5%, N = 8), pulmonary (52.0±7%, N = 10) and mesenteric arteries (76±4%, N=11). Niclosamide also impaired rat cortical pericyte constriction by 40.9±9.5% (N = 8) after exposure to endothelin-1 (10 nM). Unlike Ani9, a selective TMEM16A inhibitor, niclosamide reduced isolated artery responses to increased extracellular K⁺. Patch-clamp recordings of isolated smooth muscle cells demonstrated that niclosamide inhibits Ca_V currents and activates a hyperpolarising current, mediated at least in part by K⁺ ions.

Conclusions: Niclosamide is a potent vasodilator of a range of artery types and cortical cerebral capillaries. The underlying mechanism involves modulation of multiple membrane currents including Ca_v and hyperpolarising K⁺ current in addition to TMEM16A inhibition.