The Effect of Brimonidine on Transepithelial Resistance in a Human Retinal Pigment Epithelial Cell Line

PURPOSE: To investigate the effects of brimonidine, an alpha-2-adrenergic agonist, on barrier function in ARPE-19 cells by measuring transepithelial resistance (TER). METHODS: ARPE-19 cells were cultured into a confluent monolayer on a microporous filter. Brimonidine was added to the apical medium, and the barrier function of the cells was evaluated by measuring TER. A subset of cells was treated under hypoxic conditions, and the TER changes observed upon administration of brimonidine were compared to those observed in cells in normoxic conditions. RESULTS: The ARPE cell membrane reached a peak resistance of 29.1+/-7.97 Omega cm2 after four weeks of culture. The TER of the cells treated under normoxic conditions increased with brimonidine treatment; however, the TER of the cells treated under hypoxic conditions did not change following the administration of brimonidine. CONCLUSIONS: Barrier function in ARPE-19 cells increased with brimonidine treatment. Understanding the exact mechanism of this barrier function change requires further investigation.

Potentiation of responses to UK-14304 in rat isolated common carotid artery by angiotensin

The selective [alpha 2] -adrenoceptor agonist UK-14304 produces a small vasoconstrictor response in the rat isolated carotid artery. The purpose of the work presented here was to investigate whether stimuli that produce submaximal contraction would potentiate responses to UK-14304. Male Wistar rats were killed by overdose with pentobarbitone sodium, after which the left and right common carotid arteries were removed. The rings of arteries 3-4 mm in length were cut from each vessel and then mounted in 10 mL isolated organ bath, bathed in Krebs maintained at 37°C and gassed with 95% 02 plus 5% CO[2] The preparations were allowed to equilibrate for an hour. Cumulative concentration-response curves [CCRC] were constructed in a cumulative manner by increasing the concentration of the agonists in half-log increments. When antagonists were used, the preparations were incubated at least for 45 minutes with the drugs prior to the onset of a second CCRC. Angiotensin II [All] and other contracting factors were added approximately 10-15 min prior to the onset of CCRC to an agonist. After inducing tone with low concentrations of the thromboxane A[2] mimetic agent U-46619 [1nM], 5HT [0.5-1 [micro] M] and phenylephrine [10 [micro] M], exposure of the preparation to UK-14304 resulted in concentration dependent contractions to this agonist. The sensitivity and maximum response of the preparation to UK-14304 were not changed. Inducing tone with AII [0.01 [micro] M] produced a significant leftward shift in the CCRC to UK-14304 [p<0.05]. Thus submaximal contraction with AII [0.01 [micro] M],increased responses significantly, but inducing tone with phenylephrine, U-46619 and 5HT had no effect on responses to UK-14304. The [alpha] -adrenoceptor antagonists prazosin and rauwolscine were examined to see whether UK-14304's main action in the presence of All remained via [alpha 1]. The potentiated responses were prazosin sensitive and rauwolscine resistant, indicating an increasing effect mediated by [alpha 1] -adrenoceptotrs

Role of midbrain ventro-lateral tegmental area (VTA) adrenergic mechanisms in facilitation of hypothalamically-induced predatory attack behaviour.

Bipolar concentric electrodes were implanted in extreme lateral regions of hypothalamus having coordinates (A12.5 mm, L 3.5-3.7 mm, V 3.0-3.7 mm). These sites were electrically stimulated using biphasic square wave pulses (1 ms, 60 Hz) at a current strength ranging from 300-800 microA to evoke predatory attack on an anaesthetized rat. At lower current strengths of 300 microA only altertness with pupillary dilatation was produced. Gradual increase in the current strength led to recruitment of somatic and affective components and a full blown predatory attack on a rat was produced at a mean current strength of 700 microA. A scoring system allowed the construction of stimulus response curve, which remained fairly constant when repeated over a period of 3-4 weeks. In ventrolateral tegmental area (VTA), bilateral microinjections of norepinephrine (NE 10 micrograms in 0.5 microliter saline, pH 7.4) lowered the mean threshold current strength to 100 microA while predatory attack was produced at 500 microA. Clonidine (5 micrograms in 0.5 microliter propylene glycol, pH 7.4) an alpha-2 agonist similarly lowered the mean threshold to 100 microA and predatory attack threshold to 400 microA. The effects of clonidine appeared within 20 min of microinjection and persisted up to 6 hr. Yohimbine, an alpha-2 antagonist (4 micrograms in 0.5 microliter propylene glycol. pH 7.4) when microinjected into the same locus (VTA), completely blocked predatory attack behaviour for 3 days, the peak period of the blocking effects were between 3-8 hr, after microinjection. Isoproterenol (beta agonist), propranolol (beta blocker), prazosin (alpha-1 antagonist) and phenoxybenzamine (alpha antagonist) failed to produce any effect. Normal saline and propylene glycol in similar volumes served as controls. The excitatory effects of NE and clonidine and inhibitory effects of Yohimbine were significant at P < 0.01 and P < 0.05 respectively with Wilcoxon's signed rank test. The present study indicates the involvement of alpha-2 adrenoceptive mechanisms operating at the midbrain (VTA) level in the elicitation of predatory attack from lateral hypothalamus.

Multifactorial control of water and saline intake: role of alpha2-adrenoceptors

Water and saline intake is controlled by several mechanisms activated during dehydration. Some mechanisms, such as the production of angiotensin II and unloading of cardiovascular receptors, activate both behaviors, while others, such as the increase in blood osmolality or sodium concentration, activate water, but inhibit saline intake. Aldosterone probably activates only saline intake. Clonidine, an alpha2-adrenergic agonist, inhibits water and saline intake induced by these mechanisms. One model to describe the interactions between these multiple mechanisms is a wire-block diagram, where the brain circuit that controls each intake is represented by a summing point of its respective inhibiting and activating factors. The alpha2-adrenoceptors constitute an inhibitory factor common to both summing points.

Studies on the platelet alpha-adrenoceptor subtypes in diabetes mellitus.

Studies on the platelet alpha-adrenoceptor subtypes were carried out in 19 subjects with treated diabetes mellitus and 15 normal age and sex-matched controls. By utilising selective antagonists, it was noted that all the normal human platelets exhibited the alpha-2 adrenoceptor. 7 diabetics (37%) expressed both the alpha-1 and alpha-2 adrenoceptors and diabetic complications along with hypertension were most common in this group. Another 7 diabetics (37%) expressed the alpha-2 receptor only and diabetic complications were minimum in this group. Interestingly, 5 diabetics (26%) did not express either the alpha-1 or alpha-2 receptor and these patients occupied an intermediate position with regard to diabetic complications. Thus, it was concluded that platelet alpha-1 adrenoceptors perhaps indicated a poor prognosis in diabetes mellitus, opening up future scope for work in this area.