Photosensitization of oesophageal smooth muscle by 3-NO2-1,4-dihydropyridines: Evidence for two cyclic GMP-dependent effector pathways
Calcium Channel Agonists
Calcium Channel Blockers
1. Photoactivated mechanical responses that resulted from exposure to 3-NO2-1,4-dihydropyridines (3-NO2-DHP5) or NO-donors were examined in rat isolated oesophageal smooth muscle with a view to determining the role of calcium and cyclic GMP. 2. Isometric contractile force was recorded in preparations bathed in normal Tyrode or 110 mM K(+)-depolarizing solution. Exposure to (+)-PN 202791, (+/-)-Bay K 8644 and (-)-PN 2020791 or the photodegradable NO-donors, sodium nitroprusside (SNP), streptozotocin (STZ) and sodium nitrite photosensitized precontracted tunica muscularis mucosae preparations in a concentration-dependent fashion. Photosensitizing potency followed the order: (+/-)-PN 202791 > (+/-)-Bay K 8644 > (-)-PN 202791 > SNP > STZ > NaNO2. 3. A low amplitude, slow photorelaxation (slope: 1 mg s-1) was obtained with the L-channel antagonists (-)-PN 202791 and (+)-Bay K 4407. Photosensitization by the agonist enantiomers (+)-PN 202 791 and (-)-Bay K 5407, as well as racemic Bay K 8644, was mimicked by NO donors and showed at least three different components, consisting of (i) a fast relaxation (slope: 140 mg s-1), (ii) a fast "off-contraction', and (iii) a delayed slow relaxation. The fast components, but not the delayed slow relaxation, were abolished by blockade of L-type voltage-operated calcium channels, chelation of extracellular calcium and skinning of the plasmalemma, suggesting their mediation by a process linked to calcium entry through L-channels. 4. Both cyclopiazonic acid (3-30 microM) and ryanodine (30 microM) inhibited the fast response. This inhibition was accelerated in the presence of extracellular calcium and resembled that seen in tissues exposed to the calcium ionophore A 23187 (1 microM). In calcium depleted tissues, cyclopiazonic acid (3 microM) prevented restoration of the cis-dioxolane-induced contraction following re-exposure to a calcium containing high K+ buffer, but failed to inhibit the photoresponse. 5. Both the fast and slow relaxations were potentiated by zaprinast (10 microM) and inhibited by LY B3583 (10 microM). However, in calcium-depleted, calyculin A-precontracted preparations only the slow relaxation was evident. 6. The present results support the conclusion that: (i) functional L-channels are required for the expression of the fast components of the 3-NO2-DHP- or NO-donor-induced photoresponse, (ii) NO photorelease followed by activation of soluble guanylyl cyclase is responsible for the photosensitizing activity of 3-NO2-DHPs and (iii) regulation of the contractile proteins via cyclic GMP-dependent phosphorylation may underlie the slow relaxation.