Differential changes of adrenoceptor‐ and muscarinic receptor‐linked prostacyclin synthesis by the aorta and urinary bladder of the diabetic rat

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Abstract
1 The effect of experimental diabetes mellitus (DM; hyperglycaemic, non-ketototic; 2 months duration) in the rat on receptor-linked prostacyclin (PGI2) synthesis (measured as 6-oxo-PGF1α by radioimmunoassay) was studied in the aorta and urinary bladder using adrenaline, angiotensin II (AII) and acetylcholine (ACh). Signal transduction systems were studied via stimulation of PGI2 synthesis with phorbol ester dibutyrate (PDBU; a protein kinase C activator [PKC]), Ca2+ ionophore A23187 (A23187) and thapsigargin (both elevate intracellular Ca2+, activating phospholipase A2 [PLA2]) and arachidonate (AA; substrate for PGI2 synthesis). 2 In response to adrenaline, AII and phorbol ester, aortic PGI2 release was markedly reduced (all > 75%) in diabetic rats compared to controls. EC50s of the dose-response curves for adrenaline, AII and PDBU were also markedly increased in aortae from DM rats compared to controls. Although there was decreased output of PGI2 in response to A23187 by aortae from diabetic rats compared to controls, there was no difference in the EC50s (mean ± s.e.mean: diabetic, 2.7 ± 0.2 × 10−6 M; controls 2 ± 0.18 × 10−6 m). There were no differences in PGI2 release (or in the EC50s) in response to thapsigargin or AA between aortae from diabetic and control rats. 3 In the urinary bladder, there was a marked increase in PGI2 output in response to ACh and a marked decrease in EC50s for the ACh-PGI2 dose-response curves in diabetic rats (EC50= 5.8 ± 0.32 × 10−7 m) compared to controls (EC50 = 2.2 ± 0.15 × 10−6 m). Although there was an increase in PGI2 output in the urinary bladders from diabetic rats in response to A23187, there were no differences in the EC50s (control, 1.8 ± 0.2 × 10−6 m; diabetic, 1.1 ± 0.15 × 10−6 m). In the urinary bladders, there were no differences in PGI2 output (or the EC50s) in response to PDBU, thapsigargin or AA between diabetic or control rats. 4 These data indicate that: (i) reduced PGI2 synthesis coupled to adrenoceptors and AII receptors in the aortae of diabetic rats may be due to diminished PKC activity and not to changes in receptor density and/or affinity, Ca2+ stores, PLA2, cyclo-oxygenase or PGI2 synthase; (ii) the diametrically opposite effect of DM on ACh-stimulated PGI2 synthesis is not due to an increase in PKC activity, but possibly to an increase in muscarine receptor number and/or affinity; (iii) changes in receptor-linked PGI2 synthesis are not ubiquitous in experimental DM and may be organ-specific.