Proceedings of The Physiological Society
University of Cambridge (2004) J Physiol 555P, C36
Altered cotransmitter contributions to sympathetic vasoconstriction of tail artery in streptozotocin diabetic rats
A. Donnelly, S. Roe, C.N. Scholfield and C.D. Johnson
Department of Physiology, Queen's University, Belfast BT9 7BL, UK
Diabetes mellitus is associated with numerous cardiovascular complications including autonomic neuropathy, leading to orthostatic hypotension. Changes taking place in sensitivity to exogenous noradrenaline are well documented (Weber et al. 1996) but contributions of other sympathetically released cotransmitters are unclear. In this study we show that vascular responses mediated by ATP in the rat tail artery show distinct changes in the diabetic state.
Sprague-Dawley rats (8 week old, male) were made diabetic by intraperitoneal injection (60 mg kg-1) of streptozotocin and maintained for a further 12 weeks in accordance with UK legislation. Animals were killed by cervical dislocation. Injected animals having a blood glucose of less than 10 mM/l were used a controls (n = 9, 492 ± 26 g, mean ± S.E.M.; blood glucose 7 ± 1 mM) while those with higher values were deemed to be diabetic (n = 12, 275 ± 12 g, blood glucose 39 ± 2 mM). Tail arteries were excised, endothelium removed and rings cut into 3-5 mm lengths. Isometric contractions were measured. Noradrenaline (0.1 nM-100 µM) and ATP (1 nM-10 µM) were bath applied and dose-response curves constructed and responses examined to either 60 mM KCl or electrical field stimulation by trains of 1-100 impulses (at 20 Hz, 1 ms pulses, supra-maximal voltage). Electrically-evoked responses were examined in the absence or presence of non-specific α-adrenoceptor antagonist, phentolamine (1 µM), or P2 purinoceptor antagonist, suramin (100 µM). Electrically-evoked responses were abolished in the presence of tetrodotoxin (1 µM) or guanethidine (10 µM), indicating their sympathetic origin.
Bath applied noradrenaline and ATP were both more potent in diabetic than non-diabetic arteries (diabetic vs. non-diabetic, ED50 for noradrenaline: 0.28 ± 0.07 vs. 2.93 ± 1.0 µM: P < 0.005, unpaired t test). In contrast, there were no differences in KCl constrictions between diabetic and non-diabetic rats (0.94 ± 0.2 g and 0.85 ± 0.07 g respectively; unpaired t test) suggesting the enhanced vasoconstrictor responses were not due to changes in smooth muscle contractility. Nerve stimulation produced contractions which were similar in both diabetic and non-diabetic arteries. In non-diabetic arteries, these were reduced by suramin which was dependent on train impulse number, ie, 58 ± 12 % depression with 2 pulses and 27 ± 13 % 100 impulses). However, in diabetic arteries, depression with suramin was greater for longer pulse trains (depressions of 54 ± 12 % and 68 ± 10 % for 2 and 100 pulse trains; one way ANOVA and SNK, diabetic vs. non-diabetic). There were also depressions with phentolamine which was similar for both groups for impulse trains >4 (one way ANOVA).
Thus, we have shown that in diabetic rats, responses to sympathetic activation are preserved but there is a shift towards a greater contribution made by ATP which is dependent on pulse train parameters.
AD was funded by a Physiological Society Summer Vacationship
Where applicable, experiments conform with Society ethical requirements