Downregulation of propranolol-sensitive beta-adrenoceptor signaling after inhibition of nitric oxide synthesis

Abstract

The beta-adrenoceptor agonist, isoprenaline, elicits vasodilation and tachycardia in anesthetized rats via activation of propranolol-sensitive beta1- and beta2-adrenoceptors and also by propranolol-insensitive beta1- and beta3-adrenoceptors. The aim of this study was to determine whether the relative contribution of propranolol-sensitive and -insensitive beta-adrenoceptors to the changes in heart rate (HR) and vascular resistances elicited by isoprenaline is altered after blockade of nitric oxide (NO) synthase, in pentobarbital-anesthetized rats. The hemodynamic responses elicited by isoprenaline (0.1 and 0.5 microg kg(-1), i.v.) were determined before and after injection of saline or the NO synthase inhibitor, N(G)-nitro-L-arginine methylester (L-NAME, 50 micromol kg(-1), i.v.), and again after injection of the beta1- and beta2-adrenoceptor antagonist, propranolol (1 mg kg(-1), i.v.). The responses elicited by the above doses of isoprenaline were also determined before and during infusion of the alpha1-adrenoceptor agonist, phenylephrine (3 microg kg(-1) min(-1), i.v.), and again 15-20 min after injection of propranolol (1.0 mg kg(-1), i.v.). Both doses of isoprenaline elicited tachycardia and reductions in vascular resistances. Propranolol eliminated the responses elicited by the lower dose of isoprenaline and substantially diminished the responses elicited by the higher dose of the beta1-, beta2- and beta3-adrenoceptor agonist. The maximal vasodilator responses elicited by both doses of isoprenaline were not diminished whereas the maximal increases in HR were higher after injection of L-NAME. The ability of propranolol to diminish the hemodynamic actions of isoprenaline was substantially diminished in L-NAME-treated rats, whereas propranolol retained its potency in rats that received an equi-pressor infusion of the alpha1-adrenoceptor agonist, phenylephrine. The finding that the maximal vasodilator responses elicited by isoprenaline were not diminished by L-NAME suggests that the vasodilation elicited by this drug was due to direct activation of beta-adrenoceptors on vascular smooth muscle and that the full compliment of isoprenaline-sensitive receptors was not changed after inhibition of NO synthesis. However, these results suggest that the activities of propranolol-sensitive beta-adrenoceptors are downregulated, whereas propranolol-insensitive beta-adrenoceptors are upregulated upon the loss of exposure to endothelial nitrosyl factors.

Figure 1

Summary of the maximal changes in HR elicited by isoprenaline (0.1 μg kg⁻¹, i.v.) before and after injection of saline or L-NAME (LNM, 50 μmol kg⁻¹, i.v.) and again after an injection of propranolol (prop, 1 mg kg⁻¹). There were six rats in each group. All values are mean±s.e.m. Statistical symbols for resting HR. ^*P<0.05, significant change from Pre. ^†P<0.05, Post-propranolol versus post-saline or post-L-NAME. Statistical symbols for isoprenaline-induced changes in HR. ^*P<0.05, significant difference from Pre. ^†P<0.05, Post-L-NAME versus post-saline, and post-L-NAME+propranolol versus post-saline+propranolol. The numbers of rats and doses of drugs are the same for Figures 2, 3, 4, 5, 6, 7, 8, 9 and 10.

Figure 2

Summary of the maximal changes in HR elicited by isoprenaline (0.5 μg kg⁻¹) before and after injection of saline or LNM and again after injection of propranolol (prop). All values are mean±s.e.m. Symbols for resting HR. ^*P<0.05, significant difference from Pre. ^†P<0.05, Post-propranolol versus post-saline or post-L-NAME. Symbols for isoprenaline-induced changes in HR. ^*P<0.05, significant difference from Pre. ^†P<0.05, post-L-NAME versus post-saline and post-L-NAME+propranolol versus post-saline+propranolol.

Figure 3

Summary of the maximal changes in MAP elicited by isoprenaline (0.1 μg kg⁻¹) before and after injection of saline or LNM and again after injection of propranolol (prop). All values are mean±s.e.m. Symbols for resting MAP. ^*P<0.05, significant difference from Pre. ^†P<0.05, post-propranolol versus post-saline or post-L-NAME. Symbols for isoprenaline-induced changes in MAP. ^*P<0.05, significant change from Pre. ^†P<0.05, post-L-NAME versus post-saline and post-L-NAME+propranolol versus post-saline+propranolol.

Figure 4

Summary of the maximal changes in MAP elicited by isoprenaline (0.5 μg kg⁻¹) before and after injection of saline or LNM and again after injection of propranolol (prop). All values are mean±s.e.m. Symbols for resting MAP. ^*P<0.05, significant difference from Pre. ^†P<0.05, post-propranolol versus post-saline or post-L-NAME. Symbols for isoprenaline-induced changes in MAP. ^*P<0.05, significant change from Pre. ^†P<0.05, post-L-NAME versus post-saline and post-L-NAME+propranolol versus post-saline+propranolol.

Figure 5

Summary of the maximal changes in HQR elicited by isoprenaline (0.1 or 0.5 μg kg⁻¹) before and after injection of saline or LNM and again after injection of propranolol (prop). All values are mean±s.e.m. Symbols for resting HQR. ^*P<0.05, significant difference from Pre. ^†P<0.05, post-propranolol versus post-saline or post-L-NAME. Symbols for isoprenaline-induced changes in HQR. ^*P<0.05, significant change from Pre. ^†P<0.05, post-L-NAME versus post-saline and post-L-NAME+propranolol versus post-saline+propranolol.

Figure 6

Summary of the maximal changes in MR elicited by isoprenaline (0.1 and 0.5 μg kg⁻¹) before and after injection of saline or LNM and again after injection of propranolol (prop). All values are mean±s.e.m. Symbols for resting MR. ^*P<0.05, significant difference from Pre. ^†P<0.05, Post-propranolol versus post-saline or post-L-NAME. Symbols for isoprenaline-induced changes in MR. ^*P<0.05, significant change from Pre. ^†P<0.05, post-L-NAME versus post-saline and post-L-NAME+propranolol versus post-saline+propranolol.

Figure 7

Summary of the maximal changes in RR elicited by isoprenaline (0.1 and 0.5 μg kg⁻¹) before and after injection of saline or LNM and again after injection of propranolol (prop). All values are mean±s.e.m. Symbols for resting RR. ^*P<0.05, significant difference from Pre. ^†P<0.05, post-propranolol versus post-saline or post-L-NAME. Symbols for isoprenaline-induced changes in RR. ^*P<0.05, significant change from Pre. ^†P<0.05, post-L-NAME versus post-saline and post-L-NAME+propranolol versus post-saline+propranolol.

Figure 8

Summary of the maximal changes in HR elicited by isoprenaline (0.1 and 0.5 μg kg⁻¹) before and during infusion of phenylephrine (3 μg kg⁻¹min⁻¹) and again upon injection of propranolol (prop). All values are mean±s.e.m. Symbols for resting HR. ^*P<0.05, significant difference from Pre. ^†P<0.05, Post-propranolol versus post-saline or post-L-NAME. Symbols for isoprenaline-induced changes in HR. ^*P<0.05, significant change from Pre. ^†P<0.05, post-L-NAME versus post-saline and post-L-NAME+propranolol versus post-saline+propranolol.

Figure 9

Summary of the maximal changes in MAP elicited by isoprenaline (0.1 and 0.5 μg kg⁻¹) before and during infusion of phenylephrine (3 μg kg⁻¹min⁻¹) and again after injection of propranolol (prop). All values are mean±s.e.m. Symbols for resting MAP. ^*P<0.05, significant difference from Pre. ^†P<0.05, post-propranolol versus post-saline or post-L-NAME. Symbols for isoprenaline-induced changes in MAP. ^*P<0.05, significant change from Pre. ^†P<0.05, post-L-NAME versus post-saline and post-L-NAME+propranolol versus post-saline+propranolol.

Figure 10

Summary of the maximal changes in HQR elicited by isoprenaline (0.1 and 0.5 μg kg⁻¹) before and during infusion of phenylephrine (3 μg kg⁻1 min⁻¹) and again after injection of propranolol (prop). All values are mean±s.e.m. Symbols for resting HQR. ^*P<0.05, significant difference from Pre. ^†P<0.05, Post-propranolol versus post-saline or post-L-NAME. Symbols for isoprenaline-induced changes in HQR. ^*P<0.05, significant change from Pre. ^†P<0.05, post-L-NAME versus post-saline and post-L-NAME+propranolol versus post-saline+propranolol.