Unique properties of muscularis mucosae smooth muscle in guinea pig urinary bladder

Abstract

The muscularis mucosae, a type of smooth muscle located between the urothelium and the urinary bladder detrusor, has been described, although its properties and role in bladder function have not been characterized. Here, using mucosal tissue strips isolated from guinea pig urinary bladders, we identified spontaneous phasic contractions (SPCs) that appear to originate in the muscularis mucosae. This smooth muscle layer exhibited Ca 2ϩ waves and flashes, but localized Ca 2ϩ events (Ca 2ϩ sparks, purinergic receptor-mediated transients) were not detected. Ca 2ϩ flashes, often in bursts, occurred with a frequency (ϳ5.7/min) similar to that of SPCs (ϳ4/min), suggesting that SPCs are triggered by bursts of Ca 2ϩ flashes. The force generated by a single mucosal SPC represented the maximal force of the strip, whereas a single detrusor SPC was ϳ3% of maximal force of the detrusor strip. Electrical field stimulation (0.5-50 Hz) evoked force transients in isolated detrusor and mucosal strips. Inhibition of cholinergic receptors significantly decreased force in detrusor and mucosal strips (at higher frequencies). Concurrent inhibition of purinergic and cholinergic receptors nearly abolished evoked responses in detrusor and mucosae. Mucosal SPCs were unaffected by blocking smallconductance Ca 2ϩ -activated K ϩ (SK) channels with apamin and were unchanged by blocking large-conductance Ca 2ϩ -activated K ϩ (BK) channels with iberiotoxin (IbTX), indicating that SK and BK channels play a much smaller role in regulating muscularis mucosae SPCs than they do in regulating detrusor SPCs. Consistent with this, BK channel current density in myocytes from muscularis mucosae was ϳ20% of that in detrusor myocytes. These findings indicate that the muscularis mucosae in guinea pig represents a second smooth muscle compartment that is physiologically and pharmacologically distinct from the detrusor and may contribute to the overall contractile properties of the urinary bladder. calcium imaging; large-conductance calcium-activated potassium channel; potassium channels; calcium flashes; spontaneous phasic contractions THE URINARY BLADDER is a hollow, muscular organ that serves two functions: urine storage and elimination. The bladder wall is composed of detrusor smooth muscle, which accounts for a large fraction of the bladder mass, and a layer of transitional epithelial cells known as the urothelium, which lines the luminal surface of the urinary bladder. The detrusor smooth muscle has been extensively characterized and is clearly responsible for the majority of the contractile properties of the urinary bladder (1, 69). The urothelium, which was originally thought to simply act as a passive, impermeable barrier that prevents the movement of solutes from the urine into the bloodstream, is now recognized to possess a broad range of sensory and signal transduction functions that greatly influence bladder physiology (for a review, see Ref. 5).The detrusor exhibits two primary modes of contractile behavior: nerve-evoked contractions and spontaneo...

Ethanol-mediated relaxation of guinea pig urinary bladder smooth muscle: involvement of BK and L-type Ca2+ channels

“…Paxilline inhibited currents at 0 mV and above (P Ͻ 0.05, Fig. 1B) consistent with the previous observations in DSM cells showing contribution of BK channels at voltages of ϳ0 to ϩ10 mV and above (19,22). For instance, at ϩ40 and ϩ110 mV the mean current density value significantly decreased to 2.18 Ϯ 1.28 pA/pF and to 4.18 Ϯ 5.69 pA/pF (n ϭ 6, N ϭ 6, P Ͻ 0.05), respectively.…”

Functional Innervation of Guinea-Pig Bladder Interstitial Cells of Cajal Subtypes: Neurogenic Stimulation Evokes In Situ Calcium Transients

“…SMC bundles within the mucosal tissues may have been derived from the muscularis mucosa [15], [16] and/or underlying superficial detrusor bundles. It is difficult to be certain whether the SMC bundles studied were from the muscularis mucosa or superficial detrusor as the muscularis mucosa layer in guinea-pig bladder is discontinuous.…”

Central role of the BK channel in urinary bladder smooth muscle physiology and pathophysiology

“…Another study from Fry's group suggests the absence of functional BK channels in guinea pig bladder trigone based on the lack of Ca 2ϩ responses to iberiotoxin (applied at lower concentrations of 50 nM) in single trigonal smooth muscle cells (122). The BK channel current density in muscularis mucosae is only about 20% of that seen in UBSM (49), further confirming that BK channels are confined primarily to UBSM cells.…”