The specific role of dopamine D2, D3 and D4 receptor subtypes in quinpirole-induced inhibition of the cardioaccelerator sympathetic outflow in diabetic pithed rats

C. M. Villalón, E. Rivera-Mancilla, A. H. Altamirano-Espinoza, G. Manrique-Maldonado, B. Villanueva-Castillo. Pharmacobiology, Cinvestav-Coapa, Mexico City, MEXICO.

Introduction: The vasopressor sympathetic outflow is altered in diabetic subjects1. This involves abnormalities in central and peripheral catecholaminergic systems (including dopamine)2. Since quinpirole-induced inhibition of the cardioaccelerator sympathetic outflow in normoglycemic pithed rats involves activation of dopamine D2, but not D3 or D4, receptor subtypes3, this study investigated in diabetic pithed rats the pharmacological profile of quinpirole-induced inhibition of the cardioaccelerator sympathetic outflow.

Method: Ninety male Wistar rats (280-300 g) were pretreated with streptozotocin (50 mg/kg, i.p.; n=70) or vehicle (1 ml/kg citrate buffer, i.p.; n=20) and kept for 4 weeks. After pentobarbital anaesthesia (60 mg/kg; i.p.) the animals were pithed, artificially ventilated with room air (56 strokes/min, stroke volume: 20 ml/kg), and prepared for selective preganglionic spinal stimulation (C7-T1; 0.03-3 Hz; 50 V; 2 ms pulses) of the cardioaccelerator sympathetic outflow; or i.v. bolus of noradrenaline (0.03-3 µg/kg). The carotid artery was cannulated for measurement of blood pressure and heart rate. Before electrical stimulation, all animals received i.v. gallamine (25 mg/kg; to avoid muscular twitching) and desipramine (50 µg/kg; to amplify the sympatho-inhibitory responses at lower stimulation frequencies)3. Body temperature was kept at 37°C by a lamp. Data analysis included 2-way repeated-measures ANOVA+Student-Newman-Keuls test. Statistical significance (*) was accepted at P<0.05. Our institutional ethics committee approved our experiments (protocol number 0139-15).

Results: Electrical stimulation and i.v. bolus of noradrenaline in normoglycemic and diabetic pithed rats produced frequency-dependent and dose-dependent tachycardic responses (not shown). These responses in diabetic rats were: (i) unchanged during i.v. infusions of 0.02 ml/min saline; and (ii) dose-dependently inhibited during i.v. infusions of 3 and 10 µg/kg.min quinpirole (not illustrated). Moreover, the cardiac sympatho-inhibition by 10 µg/kg.min quinpirole in diabetic rats (Fig. 1), which did not affect the tachycardic responses to exogenous noradrenaline (not shown) was: (i) unchanged after vehicles (i.v.); and (ii) abolished after i.v. administration of the antagonists L-741,626 (D2; 300 µg/kg), SB-277011-A (D3; 300 µg/kg) or L-745,870 (D4; 100 µg/kg). The doses of these antagonists, which had no effect per se (P>0.05) on the sympathetically-induced tachycardic responses, were high enough to completely block their respective receptors in pithed rats3.

Conclusion: Quinpirole-induced cardiac sympatho-inhibition in diabetic pithed rats involves activation of dopamine D2, D3 and D4 receptor subtypes.

References:

1. Altamirano-Espinoza AH et al. (2015). Basic Clin Pharmacol Toxicol 117: 31-38.

2. Gallego M et al. (2003). Physiol Res 52: 735-741.

3. Altamirano-Espinoza AH et al. (2013). Br J Pharmacol 170: 1102-1111.

Fig. 1. Effect of vehicles or antagonists on quinpirole-induced cardiac sympatho-inhibition in diabetic pithed rats (n=5 each subgroup)