title: Selective inhibition of phosphodiesterase 1 relaxes urinary bladder smooth muscle: role for ryanodine receptor-mediated BK channel activation.
abstract: The large conductance voltage- and Ca(2+)-activated K(+) (BK) channel is a major regulator of detrusor smooth muscle (DSM) excitability and contractility. Recently, we showed that nonselective phosphodiesterase (PDE) inhibition reduces guinea pig DSM excitability and contractility by increasing BK channel activity. Here, we investigated how DSM excitability and contractility changes upon selective inhibition of PDE type 1 (PDE1) and the underlying cellular mechanism involving ryanodine receptors (RyRs) and BK channels. PDE1 inhibition with 8-methoxymethyl-3-isobutyl-1-methylxanthine (8MM-IBMX; 10 μM) increased the cAMP levels in guinea pig DSM cells. Patch-clamp experiments on freshly isolated DSM cells showed that 8MM-IBMX increased transient BK currents and the spontaneous transient hyperpolarization (STH) frequency by ∼2.5- and ∼1.8-fold, respectively. 8MM-IBMX hyperpolarized guinea pig and human DSM cell membrane potential and significantly decreased the intracellular Ca(2+) levels in guinea pig DSM cells. Blocking BK channels with 1 μM paxilline or inhibiting RyRs with 30 μM ryanodine abolished the STHs and the 8MM-IBMX inhibitory effects on the DSM cell membrane potential. Isometric DSM tension recordings showed that 8MM-IBMX significantly reduced the spontaneous phasic contraction amplitude, muscle force integral, duration, frequency, and tone of DSM isolated strips. The electrical field stimulation-induced DSM contraction amplitude, muscle force integral, and duration were also attenuated by 10 μM 8MM-IBMX. Blocking BK channels with paxilline abolished the 8MM-IBMX effects on DSM contractions. Our data provide evidence that PDE1 inhibition relaxes DSM by raising cellular cAMP levels and subsequently stimulates RyRs, which leads to BK channel activation, membrane potential hyperpolarization, and decrease in intracellular Ca(2+) levels.
authors.affiliation: Department of Pharmaceutical and Biomedical Sciences, South Carolina College of Pharmacy, University of South Carolina, Coker Life Sciences Bldg., Rm. 609D, 715 Sumter St, Columbia, SC 29208, USA.
authors.forename: Wenkuan
authors.initials: W
authors.lastname: Xin
authors.forename: Rupal P
authors.initials: RP
authors.lastname: Soder
authors.forename: Qiuping
authors.initials: Q
authors.lastname: Cheng
authors.forename: Eric S
authors.initials: ES
authors.lastname: Rovner
authors.forename: Georgi V
authors.initials: GV
authors.lastname: Petkov
mesh_terms: D015056:1-Methyl-3-isobutylxanthine / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism; D054677:Cyclic Nucleotide Phosphodiesterases, Type 1 / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D004558:Electric Stimulation; D015971:Gene Expression Regulation, Enzymologic; D006168:Guinea Pigs; D006801:Humans; D007527:Isoenzymes; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000378:metabolism*; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D009126:Muscle Relaxation / Q000187:drug effects / Q000502:physiology; D009130:Muscle, Smooth / Q000187:drug effects* / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010726:Phosphodiesterase Inhibitors / Q000737:chemistry / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*; D015398:Signal Transduction; D046509:Tissue Culture Techniques; D001743:Urinary Bladder / Q000187:drug effects / Q000502:physiology*; D014970:Xanthines / Q000494:pharmacology
chemical_list: C119531:8-methoxymethyl-3-isobutyl-1-methylxanthine; D007527:Isoenzymes; D051036:Large-Conductance Calcium-Activated Potassium Channels; D010726:Phosphodiesterase Inhibitors; D019837:Ryanodine Receptor Calcium Release Channel; D014970:Xanthines; D000242:Cyclic AMP; D054677:Cyclic Nucleotide Phosphodiesterases, Type 1; D015056:1-Methyl-3-isobutylxanthine