In Silico Pharmacological Assessment of Mibefradil in Single Detrusor Smooth Muscle Cell towards Understanding Urinary Bladder Overactivity
Authors: Mahapatra Chitaranjan , Manchanda Rohit 
Published: April 2020

OBJECTIVESEnhanced spontaneous contraction of the detrusor smooth muscle (DSM) cells, which is also known as DSM instability is associated with overactive bladder (OAB), a pathophysiological syndrome affecting millions of individuals socially. Due to adverse side effects of conventional anticholinergic drugs, researchers are focusing on novel drug compounds with high specificity. Understanding the drug effects with respect to various ion channels offers additional possibilities for safety pharmacological assessment. Here our overarching objective was to utilize computational approaches to simulate the effects of the T‐type Ca2+ channel blocker Mibefradil on the DSM cell action potential (AP) and subsequent DSM cell excitability.METHODSThe DSM cell model is described as an equivalent electrical circuit consisting of a membrane capacitance connected in parallel with a number of variable conductances representing two voltage‐gated Ca2+ (T ‐ type and L‐ type) channels, two voltage‐gated potassium (Kv, KCNQ) channels, three calcium‐dependent potassium (BK, IK and SK) channels, an ATP‐dependent potassium channel and an inward rectifying cation channel. A drug model for Mibefradil was simulated by multiplying the maximal conductance of T‐type Ca2+ channel with a scaling factor between 0 and 1 to mimic the drug concentration.KEY RESULTSIn this model, all ionic conductances were tuned to set the resting membrane potential (RMP) at − 50 mV. A synaptic input for 15 ms was injected to evoke the AP. The maximum conductance value of the T‐type Ca2+ channel (g−CaT) was set to 0.0006 S/cm2. Adding Mibefradil by 50% of its control value reduced the peak amplitude of AP and inward current. However, the addition of Mibefradil by 100% resulted in no AP and zero inward current. Representative APs are shown in Fig
1 (A) with proper legends. The T‐ type Ca2+ conductance was varied by up to ± 50 % of the control value in discrete steps to study the RMP sensitivity analysis. The normalized changes in RMP of the AP is shown in Fig
1(B). Note that RMP varies at most by up to + 10% and − 15%, indicating T‐ type Ca2+ conductance dependent DSM cell excitability. These results (Fig
1A and 1B) show that T‐type Ca2+ channels play an important role in generating AP and deactivation of T‐type Ca2+ channels decrease the excitability of DSM cell.CONCLUSIONS AND IMPLICATIONSWe investigated the ability of the Mibefradil to modulate DSM cell’s AP using our computational model which provides a virtual electrophysiological workbench. This in silico assessment showed that inhibition of T‐type Ca2+ channels hyperpolarized the RMP, eliminated the AP and reduced DSM cell excitability. This study suggests that a compound, such as Mibefradil may form a part of new pharmacological strategy in the treatment of OAB.Support or Funding InformationThis work is supported in part by Department of Biotechnology (DBT), India (grant number BT/PR12973/MED/122/47/2016).(A) Effects of Mibefradil on synaptic input based AP (B) RMP Sensitivity analysis with varying T‐ type Ca2+ conductance.Figure 1