Differential sensitivity of venular and arteriolar alpha-adrenergic receptor constriction to inhibition by hypoxia. Role of receptor subtype and coupling heterogeneity

Chronic hypoxia alters prejunctional alpha(2)-receptor function in vascular adrenergic nerves of adult and fetal sheep

The impact of development and chronic high-altitude hypoxia on the function of prejunctional alpha(2)-adrenoceptors was studied by measuring norepinephrine release in vitro from fetal and adult sheep middle cerebral and facial arteries. Blockade of prejunctional alpha(2)-adrenoceptors with idazoxan significantly increased stimulation-evoked norepinephrine release in normoxic arteries. This effect was eliminated after chronic hypoxia in cerebral arteries, with a tendency to decline in fetal facial arteries. After chronic hypoxia, the capacity to release norepinephrine declined in fetal middle cerebral arteries with a similar trend in facial arteries. Norepinephrine release was maintained in adult arteries. During development, stimulation-evoked norepinephrine release from middle cerebral and facial arteries was higher compared with adult arteries. In fetal arteries, adrenergic nerve function declined after chronic hypoxia. However, in adult arteries, adrenergic nerves adapted to chronic hypoxia by maintaining overall function. This differential adaptation of adrenergic nerves in fetal arteries may reflect differences in fetal distribution of blood flow in response to chronic hypoxic stress.

Cirrhosis of the liver and receptor-mediated function in vascular smooth muscle

Altered regulation of receptors on the vascular smooth muscle has been proposed as one of the mechanisms that may account for the vascular abnormalities in patients with cirrhosis of the liver. Impaired contractility and down-regulation of contractile receptors have been demonstrated in cirrhotic patients and animal models, although interpretation of the literature is hampered by methodological variation and conflicting results. There is little evidence, however, that receptor down-regulation is the cause of contractile dysfunction in either patients or animal models. Receptor desensitisation may contribute to impaired contraction in human arteries, but further investigation is required to confirm this possibility.

Changes in resistance vessels during hemorrhagic shock and resuscitation in conscious hamster model

The unanesthetized hamster dorsal skinfold preparation was used to monitor diameters and blood flow rates in resistance arteries (small arteries, A0: diameter, 156 +/- 23 micrometers) and capacitance vessels (small veins, V0: 365 +/- 64 micrometers), during 45 min of hemorrhagic shock at 40 mmHg mean arterial pressure (MAP) and resuscitation. A0 and V0 vessels constricted significantly to 52 and 70% of the basal values, respectively, whereas precapillary arterioles (A1-A4, 8-60 micrometers) and collecting venules (VC-VL, 26-80 micrometers) did not change or tended to dilate. Blood flow rates in the microvessels declined to <20% of the basal values. Resuscitation with shed autologous blood (SAB) showed incomplete recovery of A0 and V0 diameters even 2 h after resuscitation (71 +/- 14% and 81 +/- 18%, respectively, of basal value), whereas other vessels did not change significantly. The behavior of A0 diameter coincided with the incomplete recovery of blood flow rates in all the vessels (ca. 50%) according to Poiseuille's law, and the incomplete recovery of functional capillary density (ca. 75%). Resuscitation with 8% human serum albumin in saline (HSA) tended to show higher levels of A0 constriction and A4 dilation and lowered blood flow rates. Resuscitation with SAB restored tissue PO2 27 +/- 10 mmHg after 2 h, which was near control levels (28 +/- 5 mmHg), whereas resuscitation with HSA caused tissue PO2 to remain significantly depressed (6 +/- 2 mmHg), and flow rates were significantly lower than resuscitation with SAB. These results indicate that response of the A0 vessels is the crucial determinant of blood flow in the observed area. The constriction of A0 may help sustain MAP, and constriction of V0 may enhance blood redistribution from the skin to the vital organs under the hypotensive condition.

Cold pressor stimulus temperature and resting masseter muscle haemodynamics in normal humans

Cold pressor stimulation reportedly increases sympathetic nerve activity in human skeletal muscles. This study examined the effect of cold pressor stimulation on the resting haemodynamics of the right masseter muscle in normal individuals, using near-infrared spectroscopy. Nine healthy non-smoking males with no history of chronic muscle pain or vascular headaches participated. Their right hand was immersed in a water bath (4, 10, 15 degrees C) for exactly 1 min. Each trial lasted 7 min (1 min before, 1 min during, 5 min after stimulation) and a strictly random order was utilized for the three test temperatures and the mock trial. Masseter muscle haemoglobin concentration and oxygen saturation, as well as heart rate and blood pressure, were continuously recorded in each trial. After completing the four trials, each participant produced and sustained a 30-s maximum voluntary clench in the intercuspal position. Data across the four trials were baseline-corrected and then magnitude-normalized to the individual's highest absolute haemoglobin and oxygen signal during the 30-s maximal clenching effort. Haemoglobin and oxygen saturation increased progressively during cold pressor stimulation as the water temperature decreased (Hb, p < 0.0001; O2, p = 0.0327); very little effect was seen during the mock trial. Heart rate and blood pressure also increased progressively during the stimulation as the temperature decreased (heart rate, p = 0.0013; systolic blood pressure, p = 0.0042; diastolic blood pressure, p = 0.0156). These data suggest that cold pressor, stimulation induces a strong increase in intramuscular blood volume which appears to be due to both a local vasodilative response and increased cardiac output.

Levosimendan, a novel Ca2+ sensitizer, activates the glibenclamide-sensitive K+ channel in rat arterial myocytes

The electrophysiological effect of levosimendan, a novel Ca(2+)-sensitizing positive inotropic agent and vasodilator, was examined on rat mesenteric arterial myocytes using the patch clamp technique. Resting potential was significantly hyperpolarized with levosimendan, with an EC50 of 2.9 microM and maximal effect (19.5 +/- 3.5 mV; n = 12) at 10 microM. Levosimendan (10 microM) significantly increased the whole-cell outward current. The currents intersected close to the calculated EK (-84 mV), suggesting that the activated current was a K+ current. Hyperpolarization and stimulation of K+ current by levosimendan were not prevented by 30 microM H-7 (a non-specific inhibitor of protein kinases) and 100 nM charybdotoxin (a blocker of Ca(2+)-activated K+ channels), but were abolished by 10 microM glibenclamide. In single-channel current recording in open cell-attached patches, two types of K+ channels were observed having conductances of 26 and 154 pS. The 154 pS channels were not affected by levosimendan and glibenclamide. The 26 pS channels were evoked in one-fourth of the patches when 10 microM levosimendan (and 0.1 mM UDP) was added (at -60 mV) and channel activity was abolished by glibenclamide. The mean open probability of the 26 pS channels was 0.094 +/- 0.017 (n = 9), and the mean open time (at -60 mV) was 6.6 ms in the presence of UDP and levosimendan. Although significant hyperpolarization (4.7 +/- 1.5 mV, n = 8) was observed at 1 microM levosimendan, the same concentration did not affect Ca2+ channel currents (n = 10). In summary, levosimendan hyperpolarized the arterial myocytes, probably through activation of a glibenclamide-sensitive K+ channel. This mechanism may contribute to the vasodilating action of levosimendan.

Characterization of the alpha1B-adrenergic receptor gene promoter region and hypoxia regulatory elements in vascular smooth muscle

We previously demonstrated that alpha1B-adrenergic receptor (AR) gene transcription, mRNA, and functionally coupled receptors increase during 3% O2 exposure in aorta, but not in vena cava smooth muscle cells (SMC). We report here that alpha1BAR mRNA also increases during hypoxia in liver and lung, but not heart and kidney. A single 2.7-kb alpha1BAR mRNA was detected in aorta and vena cava during normoxia and hypoxia. The alpha1BAR 5' flanking region was sequenced to -2,460 (relative to ATG +1). Transient transfection experiments identify the minimal promoter region between -270 and -143 and sequence between -270 and -248 that are required for transcription of the alpha1BAR gene in aorta and vena cava SMC during normoxia and hypoxia. An ATTAAA motif within this sequence specifically binds aorta, vena cava, and DDT1MF-2 nuclear proteins, and transcription primarily initiates downstream of this motif at approximately -160 in aorta SMC. Sequence between -837 and -273 conferred strong hypoxic induction of transcription in aorta, but not in vena cava SMC, whereas the cis-element for the transcription factor, hypoxia-inducible factor 1, conferred hypoxia-induced transcription in both aorta and vena cava SMC. These data identify sequence required for transcription of the alpha1BAR gene in vascular SMC and suggest the atypical TATA-box, ATTAAA, may mediate this transcription. Hypoxia-sensitive regions of the alpha1BAR gene also were identified that may confer the differential hypoxic increase in alpha1BAR gene transcription in aorta, but not in vena cava SMC.