Ritanserin blocks Ca1.2 channels in rat artery smooth muscles: electrophysiological, functional, and computational studies.

Ca1.2 channel blockers or 5-HT receptor antagonists constitute effective therapy for Raynaud's syndrome. A functional link between the inhibition of 5-HT receptors and Ca1.2 channel blockade in arterial smooth muscles has been hypothesized. Therefore, the effects of ritanserin, a nonselective 5-HT receptor antagonist, on vascular Ca1.2 channels were investigated through electrophysiological, functional, and computational studies. Ritanserin blocked Ca1.2 channel currents (I) in a concentration-dependent manner (K = 3.61 µM); I inhibition was antagonized by Bay K 8644 and partially reverted upon washout. Conversely, the ritanserin analog ketanserin (100 µM) inhibited I by ~50%. Ritanserin concentration-dependently shifted the voltage dependence of the steady-state inactivation curve to more negative potentials (K = 1.58 µM) without affecting the slope of inactivation and the activation curve, and decreased I progressively during repetitive (1 Hz) step depolarizations (use-dependent block). The addition of ritanserin caused the contraction of single myocytes not yet dialyzed with the conventional method. Furthermore, in depolarized rings, ritanserin, and to a lesser extent, ketanserin, caused a concentration-dependent relaxation, which was antagonized by Bay K 8644. Ritanserin and ketanserin were docked at a region of the Ca1.2 α subunit nearby that of Bay K 8644; however, only ritanserin and Bay K 8644 formed a hydrogen bond with key residue Tyr-1489. In conclusion, ritanserin caused in vitro vasodilation, accomplished through the blockade of Ca1.2 channels, which was achieved preferentially in the inactivated and/or resting state of the channel. This novel activity encourages the development of ritanserin derivatives for their potential use in the treatment of Raynaud's syndrome.