Photosensitization of oesophageal smooth muscle by 3‐NO2‐ 1,4‐dihydropyridines: evidence for two cyclic GMP‐dependent effector pathways

Abstract

Photoactivated mechanical responses that resulted from exposure to 3‐NO2‐1,4‐dihydropyridines (3‐NO2‐DHPs) or NO‐donors were examined in rat isolated oesophageal smooth muscle with a view to determining the role of calcium and cyclic GMP. Isometric contractile force was recorded in preparations bathed in normal Tyrode or 110 mM K+‐depolarizing solution. Exposure to (+)‐PN 202 791, (±)‐Bay K 8644 and (−)−PN 2020 791 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 202 791 > (±)‐Bay K 8644>(−)−PN 202 791 > SNP > STZ > NaNO2. A low amplitude, slow photorelaxation (slope: 1 mg s−1) was obtained with the L‐channel antagonists (−)−PN 202 791 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. Both cyclopiazonic acid (3–30 μm) and ryanodine (30 μm) inhibited the fast reponse. This inhibition was accelerated in the presence of extracellular calcium and resembled that seen in tissues exposed to the calcium ionophore A 23187 (1 μm). In calcium depleted tissues, cyclopiazonic acid (3 μm) prevented restoration of the cis‐dioxolane‐induced contraction following re‐exposure to a calcium containing high K+ buffer, but failed to inhibit the photoresponse. Both the fast and slow relaxations were potentiated by zaprinast (10 μm) and inhibited by LY 83583 (10 μm). However, in calcium‐depleted, calyculin A‐precontracted preparations only the slow relaxation was evident. 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. 1995 British Pharmacological Society