The contribution of intracellular Ca²⁺ release to contraction in human bladder smooth muscle

1. The importance of Ca²⁺ release from the sarcoplasmic reticulum (SR) in excitation contraction (EC) coupling in human detrusor muscle remains controversial. In this paper the contribution of Ca²⁺ release to agonist induced contraction is assessed. 2. Dose response curves to carbachol (0.01-10 μM) were constructed before and after exposure to 200 nM Thapsigargin (Tg). Tg pre-treatment reduced the force of contraction at all agonist concentrations however, the reduction was dose dependent. At 0.1 μM the contractions were reduced to 14.5 ± 7% (mean ± s.e.mean) of controls (n = 8) while at 10 μM the contractions were only reduced to 92 ± 3% of controls (n = 10). 3. The role of external Ca²⁺ was examined by measuring the magnitude of contraction to low and high doses of agonist in the presence and absence of external Ca²⁺. With (0.1-0.3 μM) carbachol the contractions in nominally Ca²⁺ free media were 4 ± 4% of controls (n = 7) whilst with (1-10 μM) carbachol the contractions were 36 ± 8% of controls (n = 7) suggesting that at low agonist concentrations the release of Ca²⁺ has a requirement for external Ca²⁺. 4. Pre-treatment of muscle strips with the Ca²⁺ channel blocking agent diltiazem reduced the contractile responses to carbachol. Contractions induced by 0.1 μM were reduced to 29 ± 11% (P < 0.05) of controls while those activated by 10 μM were reduced to 86 ± 6% (P = 0.1) of controls (n = 4) suggesting the Ca²⁺ influx needed to activate internal store release at low agonist stimulation is through L-type Ca²⁺ channels. 5. These observations confirm the importance of thapsigargin sensitive intracellular Ca²⁺ store release in the activation of contraction of detrusor smooth muscle and suggest the overall contribution of this store depends upon the magnitude of the agonist stimulation.