TMEM16A Ca²⁺-gated Cl^– channels (CaCCs) are highly expressed in vascular smooth muscle cells and contractile pericytes, where they provide a key depolarising mechanism.  Thus, the TMEM16A channel has been proposed as a drug target for diseases of altered vessel tone including stroke, vascular dementia and (systemic, pulmonary) hypertension (1, 2). However, no therapeutic drugs interacting with the channel have yet reached clinical practice. The FDA-approved anthelmintic drug niclosamide inhibits the TMEM16A channel (3), however the effect of niclosamide on vascular smooth muscle and contractile pericytes is not fully defined. Here, we examine the effect of niclosamide on the tone of isolated rat aorta and the diameter of cerebral cortical capillaries and examine the underlying mechanism.

Niclosamide (1 mM) led to a reduction in both phenylephrine- and KCl-induced contraction of rat aortic rings, assessed through wire myography. The force generated by aortic rings in response to phenylephrine (10 µM) or KCl (100 mM) was reduced in the presence of niclosamide by 82.0±5.5% (n = 7) and 94.8±3.6% (n = 5), respectively. Niclosamide also impaired the constriction of pericytes in rat cortical brain slices by 41.6±11.3% (n = 6) after exposure to endothelin-1 (10 nM), assessed through differential interference contrast imaging.

Heterologous TMEM16A current in HEK293T cells and native CaCC currents in isolated rat aortic smooth muscle cells (SMCs) were similarly modulated by niclosamide; when measured at +100 mV, the heterologous TMEM16A and native CaCC currents were reduced by 27.8±3.9% (n = 10) and 18.7±3.7% (n = 6) in the presence of niclosamide (1 mM). However, at negative potentials (ranging from -100 to -40 mV) niclosamide activated these currents by ~2.9 folds and 3.1-folds, respectively.  The potentiation of the CaCC currents at negative potentials could not explain the niclosamide-mediated vasorelaxation, since CaCC currents are depolarising, and promote smooth muscle contraction. In SMCs, niclosamide (1 mM) significantly inhibited voltage gated Ca²⁺ currents and potentiated a hyperpolarising current; these effects are likely determinants of niclosamide-induced relaxation.

This study elucidated the effects of niclosamide on a range of ionic currents and excluded the TMEM16A channel as a mediator of niclosamide-induced relaxation in arterial smooth muscle and contractile pericytes. Since niclosamide has been proposed for drug repurposing for a variety of indications, knowledge of its molecular targets will increase our understanding of the therapeutic and possible side effects of this drug. This work also offers insight into the relative contribution of a range of ionic currents to the physiological control of vascular tone.