Adreno-muscarinic synergy in the bladder trigone: calcium-dependent and -independent mechanisms

We have recently demonstrated a strong synergy between both excitatory pathways in the trigone which--as part of the bladder base--is believed to play a key role in outflow control. The aim of this study was to reveal whether modulation of intracellular Ca2+, Ca2+]i, is mainly responsible for this synergistic effect. Intact muscle strips from the superficial trigone of male guinea-pigs were used for tension experiments. In isolated trigonal cells, Ca2+]i was measured by epifluorescence microscopy using the fluorescent Ca2+-indicator Fura-2. Phenylephrine (PE, 10 microM) augmented contractions induced by carbachol (1 microM) to 4.0+/-0.8-fold of control, while corresponding Ca2+]i levels did not exceed 1.3+/-0.2-fold of control. Furthermore, PE generated significantly greater contractions for a given rise of Ca2+]i, compared to depolarising KCl solutions. The protein kinase C inhibitor GF 109203X (5 microM) and the Rho-kinase inhibitor Y-27632 (5microM) reduced the PE contracture to 37.3+/-9.4 and 60.1+/-12.4% of control, respectively, without significantly altering the Ca2+]i transients. GF 109203X reduced the augmentation of 1microM carbachol by PE to 1.5+/-0.1-fold. Muscarinic and adrenergic receptor activation exerts a strong synergistic effect in the bladder trigone without similar changes to the Ca2+]i transient. Ca2+-sensitisation of contractile proteins is likely to play a key role in this synergism, particularly for adrenergic activation.