The role of extracellular Ca²⁺ in carbachol-induced tonic contraction of the pig detrusor smooth muscle

The role of extracellular Ca²⁺ in the toniccontractile response to muscarinic receptor stimulation was investigated in isolated detrusor smooth muscle from the pig urinary bladder. Carbachol (10^-8 -10^-5 M) produced a concentration-dependent contractile response in isolated pig detrusor smooth muscle strips consisting of an initial phasic component followed by a tonic component. During the plateau of the tonic contractions induced by carbachol at the submaximal concentration of 10^-6 M, the inhibiting effects of atropine, EGTA, nifedipine (a voltage-dependent calcium channel antagonist), H-7 [a protein kinase C (PKC) inhibitor] and YM 934 (a potassium channel opener) on the contractions were evaluated. Atropine (10^-10 -3 × 10^-8 M) concentration-dependently inhibited the tonic contractions induced by carbachol. In the same experimental conditions, EGTA (4 mM) and nifedipine (10^-9 -3 × 10^-7 M) depressed the tonic contractions in a concentration-dependent manner as did H-7 (10^-5 -3 × 10^-5 M) and YM934 (10^-8 -10^-6 M). However, H-7 (10^-5-3 × 10^-5 M) and YM934 (10^-6 M) were very weak in inhibiting the contractions induced by KCl (50 mM) in isolated pig detrusor smooth muscle strips. These results suggest that the tonic-contractile response induced by carbachol in pig detrusor smooth muscle strips is dependent mainly on depolarization of the cell membranes and an influx of extracellular Ca²⁺, and also suggest that this depolarizing response may be due to inactivation of ATP-sensitive potassium channels through muscarinic activation of PKC.