Vascular responsiveness to periarterial electrical nerve stimulation on canine ophthalmic arteries

Sympathetic activities influence blood-flow velocity and resistance in the rabbit ophthalmic artery

PURPOSE

The aim of this study was to investigate sympathetic influences on blood-flow velocity and resistance in the rabbit ophthalmic artery during the transition period from the light to the dark phase.

METHODS

Eight (8) New Zealand albino rabbits were entrained to a daily 12-h light and 12-h dark cycle. Blood-flow velocities in the ophthalmic artery were determined at -2 (baseline), 0, 2, and 4 h after the onset of darkness. Pulsed and color Doppler images of the ophthalmic artery were recorded using a 12-MHz linear ultrasound transducer. Resistive index of blood flow was calculated using the peak systolic velocity and the end diastolic velocity. Measurements of intraocular pressure (IOP) were taken, using a pneumatonometer at the same time points. Blood-flow velocity, resistive index, and IOP were also determined in the same rabbits after surgical decentralization of the ocular sympathetic nerves.

RESULTS

Compared with the baseline at -2 h, a significant increase in the resistive index of blood flow in the ophthalmic artery occurred at 2 and 4 h after the onset of darkness. Parallel elevations of IOP were observed. After the sympathetic decentralization, peak systolic and end diastolic velocities decreased and resistive indexes increased from the presurgical values in the ophthalmic artery. In the postsurgical rabbits, elevation of IOP was absent during the light-dark transition period. There was also no time-dependent increase of resistive index in the ophthalmic artery.

CONCLUSIONS

During the light-dark transition period, resistance to blood flow in the rabbit ophthalmic artery increases in parallel to IOP elevation. The removal of sympathetic activities decreases blood-flow velocity, but increases blood-flow resistance. Like the endogenous elevation of IOP, the time-dependent increase of resistance to blood flow in the rabbit ophthalmic artery during the light-dark transition period depends upon, at least partially, ocular sympathetic activities.

Adrenergic and nitrergic neurotransmitters are released by the autonomic system of the pig long posterior ciliary artery

The role played by adrenergic, muscarinic and nitric oxide putative neurotransmitters released from autonomic nerve endings onto the pig proximal long posterior ciliary artery (LPCA) was determined. The proximal LPCA in the pig usually supplies both the uveal and retinal circulations. In this study, in vitro ring segments of the artery, passively stretched and with noradrenaline-induced tone, were neurogenically stimulated (NS) using electrical field stimulation with 5-sec trains of 0.2 msec pulses. NS produced a frequency dependent contraction in all vessels which was completely abolished by 10(-6) M tetrodotoxin. 40 Hz stimulation was used throughout the study as it produced a maximal NS contraction. 10(-5) M guanethidine abolished the NS-induced contraction and revealed a NS-induced relaxation, as did the alpha adrenergic blocker, phentolamine, in vessels passively stretched. The beta adrenergic blocker, propranolol, only slightly reduced the NS-induced constriction. In vessels pre-contracted with noradrenaline, NS produced a relaxation (D) which was proportional in magnitude to the tone (C) viz. D = (0.30 +/- 0.04).C + (0.24 +/- 0.06). The muscarinic blocker, atropine, had no effect on the NS-induced relaxation, implying that it is a non-adrenergic, non-cholinergic mediated system. Incubation with Nw-nitro-L-arginine methyl ester reduced the NS-induced relaxation to 48% of its control value, a reduction which was reversed in the presence of excess L-arginine. Damage to endothelial cell function did not reduce the NS-induced relaxation. It is concluded that the autonomic innervation of the proximal LPCA releases both contraction and relaxation neurotransmitters. Contraction is mediated by an alpha adrenergic neurotransmitter. At least two neurotransmitters mediate relaxation, one of which is probably nitric oxide. There is no functional evidence for the release of beta adrenergic neurotransmitter from the sympathetic system or acetylcholine from the parasympathetic system.

Pharmacological and mechanical heterogeneity of cat isolated ophthalmociliary artery

The mechanical, histological and pharmacological properties of the isolated cat ophthalmociliary artery are reported and compared for passively stretched ring segments from three locations on the artery. Ring segments were mounted in a modified myograph system after the method of Hogestatt, Andersson and Edvinsson (1983, Acta Physiol Scand. 117, 49-61) and comparative length tension curves were measured. The most distal segment (DOA), nearest the eye, developed by far the smallest maximal tension compared to the middle segment (MOA) and the most proximal segment (POA), despite the fact that lumen diameters were similar. All segments displayed a phasic and tonic component in response to passive stretching and during activation with K(+)-Krebs. Cumulative contractile dose-response curves were measured for nine agonists: histamine (HIS), 5-hydroxytryptamine (5-HT), dopamine (DOPA), adrenaline (A), noradrenaline (NA), tyramine (TYR), phenylephrine (PHE), isoproterenol (ISOP) and xylazine (XYL) and the maximum active tension developed at a concentration of 10(-3) M was compared with that for 0.124 M K(+)-Krebs. The order of contraction magnitude in the three segments was: POA, HIS much greater than Kmax much greater than NA greater than A greater than DOPA greater than PH = TYR greater than 5-HT = ISOP greater than XYL; MOA, Kmax much greater than NA much greater than A = HIS much greater than DOPA greater than ISOP = PHE greater than TYR = 5-HT greater than XYL; DOA, Kmax much greater than NA greater than A greater than HIS = DOPA greater than ISOP greater than PHE = 5-HT greater than XYL = TYR. Pre-activation did not produce relaxation in response to histamine and isoproterenol. Testing with specific alpha 1 and beta antagonists supported the presence of alpha 1 receptors but not beta receptors. It is concluded that functional HIS, alpha 1 and 5-HT receptors are present and that the ophthalmociliary artery is heterogeneous in its response to HIS, with the POA segment producing the largest responses. The magnitude of the contractile responses to A and NA grew with increasing passive tension, whereas the response to histamine was independent of passive tension.

In vitro characterization of the mechanical properties of canine ophthalmociliary artery

The form and magnitude of K+ Krebs-induced contractures has been measured for the dog ophthalmociliary artery. Ring segments of the vessel were mounted in a myograph after the method of Hogestatt, Andersson and Edvinsson (1983, Acta Physiol. Scand. 117, 49-61). For low passive tensions the K(-)-induced contractures show both a phasic and tonic component, with the tonic tension component being the smaller. As passive tension is increased, the phasic component grows and plateaus whilst the tonic component continues to increase. For high passive tensions the phasic component disappears and the tonic component reduces in magnitude and the contracture is not fully reversible. Comparison of the length tension curves measured during K+ Krebs-induced contractures and Ca2(+)-free Krebs-induced relaxation shows that the active tension generated by this vessel increases with increasing passive tension until a peak active tension of 3.1 mN mm-1 is reached at an effective radius of 357 microns. For greater values of passive tension the active tension decreases. The calculated transmural pressure required to maintain the vessel at this optimal radius is 51.9 mmHg, which is very similar to the only available measurements of ophthalmic artery pressure, viz. 51.8 mmHg (Grunwald, Sinclair and Riva, 1981, Invest. Ophthalmol. Vis. Sci. 20, 564-6). The optimal passive stretch to apply to these ring segments for further pharmacological or physiological work is discussed.

Vascular responses of ophthalmic arteries to exogenous and endogenous norepinephrine

A participation of neuronal and tissue uptake mechanism in responses to periarterial electrical sympathetic nerve stimulation (ES) and intra-arterial applications of norepinephrine (NE) was investigated in the isolated and perfused canine ophthalmic arteries (OA), using key pharmacological drugs such as imipramine, tyramine, cocaine, cortisol and diltiazem. Results were as follows: (1) NE, KCl and ES induced marked vasoconstrictions in a dose- and frequency-related manner, but tyramine induced only a slight constriction even at large doses; (2) cortisol, an uptake2 blocker, did not significantly modify KCl- and ES-induced vasoconstrictions, but significantly enhanced NE-induced responses; (3) small doses of imipramine, a neuronal uptake1 blocker, did not modify NE-induced vasoconstrictions, but large doses decreased them. On the other hand, the ES-induced response was slightly increased by a relative small dose of imipramine; (4) cocaine, another uptake1 blocker, slightly enhanced NE- and ES-induced responses; (5) ES-induced response was reduced by not only diltiazem, but also Ca2+-free solution with EGTA (1 mM l-1). The NE-induced response was not affected by diltiazem (Ohkubo and Chiba, 1987, Exp. Eye Res. 45, 263-70), and slightly but significantly depressed by Ca2+-free solution with EGTA. These results suggest that tissue and neuronal uptake mechanisms exist in responses to ES and NE1, the response to ES was markedly dependent on Ca2+ influx to the cell membrane2, and exogenous NE may induce Ca2+ movement mainly from the intracellular store site.