Nerve-released acetylcholine contracts urinary bladder smooth muscle by inducing action potentials independently of IP3-mediated calcium release.
Nerve-released ACh is the main stimulus for contraction of urinary bladder smooth muscle (UBSM). Here, the mechanisms by which ACh contracts UBSM are explored by determining Ca(2+) and electrical signals induced by nerve-released ACh. Photolysis of caged inositol 1,4,5-trisphosphate (IP(3)) evoked Ca(2+) release from the sarcoplasmic reticulum. Electrical field stimulation (20 Hz) induced Ca(2+) waves within the smooth muscle that were present only during stimulus application. Ca(2+) waves were blocked by inhibition of muscarinic ACh receptors (mAChRs) with atropine and depletion of sarcoplasmic reticulum Ca(2+) stores with cyclopiazonic acid (CPA), and therefore likely reflect activation of IP(3) receptors (IP(3)Rs). Electrical field stimulation also increased excitability to induce action potentials (APs) that were accompanied by Ca(2+) flashes, reflecting Ca(2+) entry through voltage-dependent Ca(2+) channels (VDCCs) during the action potential. The evoked Ca(2+) flashes and APs occurred as a burst with a lag time of approximately 1.5 s after onset of stimulation. They were not inhibited by blocking IP(3)-mediated Ca(2+) waves, but by blockers of mAChRs (atropine) and VDCCs (diltiazem). Nerve-evoked contractions of UBSM strips were greatly reduced by blocking VDCCs, but not by preventing IP(3)-mediated Ca(2+) signaling with cyclopiazonic acid or inhibition of PLC with U73122. These results indicate that ACh released from nerve varicosities induces IP(3)-mediated Ca(2+) waves during stimulation; but contrary to expectations, these signals do not appear to participate in contraction. In addition, our data provide compelling evidence that UBSM contractions evoked by nerve-released ACh depend on increased excitability and the resultant Ca(2+) entry through VDCCs during APs.
Nausch
Bernhard
B
Department of Pharmacology, College of Medicine, University of Vermont, Burlington, VT 05405, USA.
Heppner
Thomas J
TJ
Nelson
Mark T
MT
D000109:Acetylcholine / Q000378:metabolism*; D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D009119:Muscle Contraction / Q000502:physiology*; D009130:Muscle, Smooth / Q000502:physiology*; D012519:Sarcoplasmic Reticulum; D001743:Urinary Bladder / Q000294:innervation / Q000502:physiology*
D015544:Inositol 1,4,5-Trisphosphate; D000109:Acetylcholine; D002118:Calcium