Functional characterization of alpha1-adrenergic receptors in experimental vein grafts

Noradrenaline stimulates cell proliferation by suppressing potassium channels via G(i/o) -protein-coupled α(1B) -adrenoceptors in human osteoblasts

BACKGROUND AND PURPOSE

Recent studies demonstrated that the sympathetic nervous system regulates bone metabolism via β(2) -adrenoceptors. Although α-adrenoceptors are also expressed in osteogenic cells, their functions in bone metabolism have been less studied. We previously demonstrated that noradrenaline suppressed potassium currents via α(1B) -adrenoceptors in the human osteoblast SaM-1 cell line. The aim of this study was to investigate the signal transduction pathway and the physiological role of noradrenaline in human osteoblasts in more detail.

EXPERIMENTAL APPROACH

To investigate signal transduction through α(1B) -adrenoceptors, we used whole-cell patch clamp recording and Ca fluorescence imaging. Potassium channels regulate membrane potential and cell proliferation activity in non-excitable cells, so we evaluated cell proliferation activity by BrdU incorporation and WST assay.

KEY RESULTS

In SaM-1 cells, bath-applied noradrenaline elevated intracellular Ca(2+) concentration and this effect was abolished by both chloroethylclonidine, an α(1B) -adrenoceptor antagonist, and U73122, a PLC inhibitor. However, the inhibitory effect of noradrenaline on whole-cell current was unaffected by U73122. In contrast, in cells pretreated with either Pertussis toxin, a G(i/o) -protein-coupled receptor inhibitor, or gallein, a Gβγ-protein inhibitor, the inhibitory effect of noradrenaline on whole-cell current was significantly suppressed. Noradrenaline-induced enhancement of cell proliferation was inhibited by CsCl, a non-selective potassium channel blocker, gallein and H89, a PKA inhibitor, but not by U73122.

CONCLUSIONS AND IMPLICATIONS

Noradrenaline facilitated cell proliferation by regulation of potassium currents in human osteoblasts via G(i/o) -protein-coupled α(1B) -adrenoceptors, not via coupling to Gq-proteins.

Pharmacological characterization of alpha1-adrenoceptors in equine digital veins

Alpha-adrenoceptors mediate contractile responses in equine digital veins (EDVs) and arteries. Vascular smooth muscle alpha(1)-adrenoceptor subtypes have been implicated in a number of conditions, such as acute equine laminitis, and are therapeutic targets for the treatment of this condition. Digital veins, rather than arteries, were investigated in the present study because they have been specifically implicated in the pathophysiology of acute laminitis. The order of potency of a series of alpha(1)-adrenoceptor-selective agonists and antagonists was determined in isolated rings of EDVs under conditions of isometric tension. A61603 was the most potent agonist, with a higher potency (76-fold greater) than phenylephrine (PHE), suggesting the presence of the alpha(1A)-adrenoceptor subtype. Prazosin (30 nm) caused competitive inhibition of the responses to A61603 and PHE, with pK(b) values of 8.05 +/- 0.28 and 8.20 +/- 0.27, respectively. In addition, the alpha(1A)-adrenoceptor antagonist, WB4101 (10 nm), also caused competitive inhibition of the responses to the two agonists, with pK(b) values of 8.37 +/- 0.16 and 8.54 +/- 0.23, respectively, confirming the presence of the alpha(1A)-adrenoceptor subtype in EDVs. The selective alpha(1D)-adrenoceptor antagonist, BMY7378 (100 nm) did not cause a significant change in the response to the agonists, giving lower pK(b) values (6.97 +/- 0.27 and 6.88 +/- 0.17 vs. A61603 and PHE, respectively). Chloroethylclonidine dihydrochloride (45 microm, 30 min), used to produce selective inactivation of alpha(1B)-adrenoceptors, caused noncompetitive inhibition of the response to PHE, but was without effect on the response to A61603. These findings indicate that EDVs possess at least two different alpha(1)-adrenoceptor populations, which are predominantly of the alpha(1A) and alpha(1B) subtypes. These data may assist in the development of more selective antagonists for therapeutic use in horses.

Characterization of dopamine-mediated relaxation in experimental vein bypass grafts

BACKGROUND

Dopamine is an endogenous inotropic agent commonly used during coronary artery surgery and in the medical therapy of a revascularized patient. In this study the responses of intimal hyperplastic vein grafts to dopamine are examined.

METHODS

The in vitro isometric tension responses to dopamine of common carotid jugular vein bypass grafts in New Zealand White rabbits were determined. The responses were compared to those obtained in the jugular vein and in the common carotid artery. Both endothelialized and denuded vessels were precontracted with prostaglandin F(2alpha) and the responses to dopamine were assessed. The contributions of nitric oxide and prostanoids to the response were also determined.

RESULTS

Each vessel showed a biphasic dose response to dopamine with relaxation at low concentrations followed by contraction at high concentrations. Dopamine relaxation in the jugular vein was endothelial independent while in the carotid artery it was endothelial dependent and decreased. The sensitivity of both vessels was significantly greater than the vein graft (6.62 +/- 0.12; P < 0. 05); however, after endothelial denudation, the sensitivity of dopamine-mediated relaxation of the vein graft (8.91 +/- 0.09) was significantly enhanced. Preincubation with L-NMMA (to block NO synthesis) inhibited vein graft relaxation to dopamine and preincubation with indomethacin (to block cyclooxygenase activity) inhibited carotid artery relaxation to dopamine. Addition of phenoxybenzamine, a broad alpha-adrenergic antagonist, enhanced dopamine relaxation in the jugular vein and depressed the relaxation in the carotid artery. There was no effect on the dopamine response in the vein graft. Jugular vein and carotid artery responded to dopamine with cholera toxin-sensitive (Galpha(s)) responses. In contrast, dopamine relaxation in the vein graft was enhanced by inhibition of Galpha(s).

CONCLUSION

Dopamine relaxation in vein grafts is mediated in part by NO but not by either prostanoids or alpha-adrenergic receptor activation. It is diminished compared to native vessels due to an endothelium-dependent, Galpha(s)-mediated pathway.