The density of beta-adrenoreceptors in cardiac muscle membranes of muskrats and guinea pigs

1. Cardiac membranes were prepared and then incubated with a range of concentrations of the beta-adrenergic ligand, dihydroalprenolol. Specific binding to the receptor was measured and receptor density and binding affinities were determined. 2. The receptor density was about 40% greater in the guinea pig heart than in the muskrat heart. The reduced beta-receptor density in the muskrat heart helps explain the previous findings of a lesser pumping ability and decreased responsiveness to isoproterenol in the muskrat compared with guinea pig hearts. 3. The decreased beta-receptor density may aid the muskrat in diving by reducing the oxygen consumption of the heart and thus prolonging diving time.     

Response of pineal beta-adrenoceptors in different breeds of sheep to immunization against selected steroids

Active immunization of Merino and Wiltshire Horn X Merino cross-bred sheep against a range of gonadal steroids has revealed that pineal beta-adrenoceptors in both breeds are sensitive to hormonal modification by androgens, but only in the less seasonal, non-shedding Merino do these receptors appear to be sensitive to regulation by estrogens. Neither breed showed sensitivity of its pineal beta-adrenoceptors to active immunization against the pineal hormone melatonin under either normal or reversed photoperiod treatment. These results (a) suggest that pineal related seasonal differences between the breeds (i.e. wool shedding and/or reproductive function) may reside in differential sensitivity of the pineal gland to regulation by specific circulating steroid hormones, and (b) indicate an absence of end-product regulation of beta-adrenoceptor-mediated pineal melatonin biosynthesis in both seasonally shedding and non-shedding sheep.     

Alloxan-induced diabetes reduces beta-adrenergic receptor number without affecting adenylate cyclase in rat ventricular membranes

We have investigated alterations in beta-adrenergic receptors and adenylate cyclase activity in myocardial membranes from normal and alloxan-treated diabetic rats. Saturation curves of [3H]dihydroalprenolol binding yielded a Bmax of 96.3 +/- 3.9 fmol/mg protein in normal membranes and 47.6 +/- 3.9 fmol/mg protein in diabetic membranes. Decreased receptor number in membranes from diabetic animals was not accompanied by alteration in receptor affinity for either antagonists or agonists to the beta-receptor. We were unable to detect any alteration in adenylate cyclase activity in similar ventricular membranes. Adenylate cyclase activity in the basal state or in the presence of sodium fluoride, guanyl-5'-yl imidodiphosphate, or isoproterenol, with or without GTP, was not altered by the alloxan-induced diabetic state. Stimulation of adenylate cyclase activity by forskolin, the novel diterpene activator, also was not altered by diabetes. The results suggest that while diabetes reduced beta-receptor number, this is not reflected in any other component of the adenylate cyclase complex.     

Chronic heart failure in the guinea pig increases cardiac alpha 1- and beta-adrenoceptors

Compared with sham-operated guinea pigs, chronic heart failure produced by aortic constriction resulted in a 79% increase in alpha 1-adrenoceptor density as measured by [3H]prazosin binding (73 +/- 15 vs. 131 +/- 20 fmol/mg protein, P < 0.05); beta-adrenoceptor number, measured by (-)-[3H]dihydroalprenolol binding, increased by 93% (28 +/- 3.4 vs. 54 +/- 5.2 fmol/mg protein, P < 0.01). For each radioligand KD values were unchanged. Thus, congestive heart failure due to chronic pressure overload results in compensatory augmentation of alpha 1- and beta-adrenoceptor numbers.     

Plasma melatonin levels in affective states

Melatonin is a major endocrine product of the pineal gland. It is produced at night when noradrenaline acts on beta-adrenergic receptors to stimulate enzymes which catalyse the formation of melatonin from serotonin. It is believed by some that nocturnal melatonin levels reflect beta-receptor function. The melatonin rhythm is also thought to be an indication of circadian rhythmicity. The nocturnal production of melatonin was studied in patients with depression and panic disorder and in control subjects. Midnight concentrations of melatonin in eleven depressed patients were significantly lower than 18 control subjects (27.1 +/- 5.1 pg/ml compared with 51.6 +/- 4.1 pg/ml; p less than 0.02, t-test). These data support previous reports of reduced melatonin synthesis in depressive illness. In the first report of patients with panic disorders, significantly lower midnight levels of melatonin were found compared with controls (28.4 +/- 6.4 pg/ml versus 51.6 +/- 4.1 pg/ml, p less than 0.02, t-test). In subsequent investigations this finding was confirmed, measuring melatonin levels over the initial phase of synthesis (i.e. 20h00 to 24h00). In these samples the melatonin rhythm also seemed to be delayed. These findings are discussed in terms of beta-receptor function and circadian rhythm alterations in affective disorders.     

Reduced number of alpha- and beta-adrenergic receptors in the myocardium of rats exposed to tobacco smoke

The concentration of alpha- and beta-adrenergic receptors--as measured by specific [3H]WB-4101 and (-)-[3H]dihydroalprenolol binding--was diminished by 60% below control values in the hearts of rats exposed to tobacco smoke. These changes in receptor numbers took place almost immediately after tobacco smoke exposure and were rapidly reversible after termination of the exposure. The dissociation constant, KD, for [3H]WB-4101 was identical in exposed (KD = 0.34 +/- 0.09 nM) and control (KD = 0.35 +/- 0.07 nM) hearts but was significantly different in the case of (-)-[3H]dihydroalprenolol binding (exposed, KD = 2.83 +/- 0.30 mM vs. control KD = 5.22 +/- 0.61 nM). For beta-receptor binding there was no significant difference between exposed and control animals in the Ki values for (-)-epinephrine, (-)-norepinephrine, (-)-alprenolol, (+/-)-propranolol or timolol. (-)-Isoproterenol, however, was found to bind with lower affinity in exposed compared with control hearts. For alpha-receptor binding there was no significant difference between control and 'smoked' animals in the Ki values for (-)-epinephrine, (-0)-norepinephrine or phentolamine. The decrease in alpha- and beta-adrenergic receptor concentration may be related to the phenomenon of receptor desensitization resulting from a release of catecholamines in rats exposed to tobacco smoke.     

Comparative studies with the enantiomers of the glycol metabolite of propranolol and their effects on the cardiac beta-adrenoceptor

The two enantiomers ((R)- and (S)-) of propranolol glycol, a metabolite of propranolol, have been synthesized, and their effects upon the beta-adrenoceptor studied by two methods. The ability of these compounds to antagonize the inotropic actions of isoprenaline was examined on spontaneously beating rat atrial preparations. Also, the effects of these enantiomers upon the binding of [3H]dihydroalprenolol to beta-receptors in rat cardiac ventricular membranes was studied. Experiments with the atria indicated that the (S)-glycol was a reversible competitive antagonist of isoprenaline with a potency approximately one thousand times lower than that of (+/-)-propranolol. In contrast, the (R)-glycol appeared to act as an irreversible antagonist, producing complex dose-response curves. The effects of these compounds to cause displacement of alprenolol binding were consistent with the organ bath data. The interaction of the (S)-glycol with the beta-receptor binding site was reversible (Ki of 27.6 +/- 4.2 microM) but less potent than that of (+/-)-propranolol (Ki of 0.99 +/- 0.07 nM). On the other hand, pretreatment of ventricular membranes with the (R)-glycol, followed by extensive washing techniques, resulted in alprenolol binding which did not regain control values, providing further evidence for an irreversible effect upon the beta-receptor. The possible significance of these pharmacological actions of the two enantiomers is discussed in terms of the in vivo metabolic pathways for propranolol.     

Alterations in cardiac beta-adrenoceptor responsiveness and adenylate cyclase system by congestive heart failure in dogs

The effects of congestive heart failure on the physiological and biochemical functions of the cardiac beta-adrenoceptor-coupled adenylate cyclase system were studied in dogs with right heart failure produced by progressive pulmonary artery constriction and tricuspid avulsion. The cardiac inotropic response to dobutamine was attenuated in congestive heart failure, as determined by the right and left ventricular dP/dt responses. Adrenergic beta-receptor density, measured by [3H]dihydroalprenolol binding, was reduced in membrane fractions of the failing right ventricle, but not in the left ventricle. The functional activity of the adenylate cyclase system was studied in vitro by measuring the net cyclic AMP production following additions of isoproterenol, 5'-guanylylimidodiphosphate (Gpp(NH)p), forskolin, or manganese chloride, which act either directly on the beta-adrenergic receptors or on one of the post-receptor components of the adenylate cyclase system. Congestive heart failure reduced the net production of cyclic AMP by isoproterenol, Gpp(NH)p, and forskolin in both the right and left ventricles, but did not alter the effect of manganese chloride. Thus, beta-receptor down-regulation is chamber-specific, occurring only in the hemodynamically stressed right ventricle. In contrast, the post-receptor defect of the adenylate cyclase system occurred in both ventricles of the heart failure dogs. This decreased activation of adenylate cyclase by beta-agonists may be responsible, at least in part, for the diminished cardiac inotropic response to catecholamines in congestive heart failure.     

Effects of chronic beta-blockade on beta-receptors and adenylate cyclase activity in the canine heart

To test the hypothesis that beta-receptors 'up-regulate' during chronic beta-blockade, myocardial beta-receptor density, affinity and functional sensitivity were examined in dogs before and after chronic beta-blockade. Membrane preparations from the left ventricle and right atrium were assayed for beta-receptors in 8 control dogs, and 8 dogs given oral nadolol (4 mg/kg) twice a day for 3 weeks. Beta-receptor density and affinity of membrane fractions were determined using 3H-dihydroalprenolol (0.5-20 nM), and Scatchard analysis. Beta-receptor densities after chronic beta-blockade were 419 +/- 28 (mean +/- SEM) and 436 +/- 47 fmol/mg protein in membranes from the left ventricle and right atrium respectively. These values were not significantly different from the left ventricle (412 +/- 25) or the right atrium (413 +/- 25) in control dogs. Beta-receptor affinities were 5.6 +/- 0.5 nM (left ventricle) and 8.5 +/- 1.1 nM (right atrium) after chronic beta-blockade, and not significantly different from control (6.9 +/- 0.7 nM, ventricle; 11.3 +/- 0.9 nM, atrium). Beta-receptor sensitivity was determined in membrane fractions from the left ventricle by analysis of the dose-response relationship between isoproterenol and the formation of c-AMP (pmol/min/mg protein). The concentration of isoproterenol that resulted in 50% of the maximal response was not significantly altered by chronic beta-blockade. Maximally stimulated adenylate cyclase with Gpp(NH)p or NaF was also unchanged by beta-blockade. These results suggest that in the canine heart, chronic beta-blockade does not lead to 'up-regulation' of beta-receptors nor catecholamine supersensitivity.     

Effect of pentoxifylline on alpha- and beta-adrenoceptor sites in cerebral cortex medial basal hypothalamus and pineal gland of the rat

The intraperitoneal injection of the methylxanthine derivative pentoxifylline (3,7-dimethyl-1-(5-oxo-hexyl)-xanthine] brought about, 3 hr later, a significant depression of alpha- and beta-adrenoceptor sites in the cerebral cortex, and of beta-adrenoceptor sites in medial basal hypothalamus and pineal gland, (assessed from the specific binding of radioactive dihydroergocryptine and dihydroalprenolol respectively). The changes in the density of binding sites were not accompanied by significant modifications of the Kd's. Sympathetic denervation of the pineal gland by superior cervical ganglionectomy (SCGx) abolished the changes of beta-adrenoceptor number in the pineal caused by pentoxifylline. The increase of alpha-adrenoceptor sites in the hypothalamus brought about by ganglionectomy was not affected by injection of pentoxifylline. Pentoxifylline did not compete in vitro for radioligand binding to brain membranes. These results suggest that methylxanthines depress brain adrenoceptor sites acutely, probably by down-regulation of receptors following the increase in catecholamine release caused by injection of the drug.     

The effect of chronic timolol in an animal model for myocardial infarction

The effect of no drug or timolol (5 mg/kg, PO, for 1, 2, or 8 weeks on postganglionic cardiac sympathetic neural discharge, blood pressure, heart rate and beta-receptor density after acute coronary occlusion of the left anterior descending artery was compared. Beta-receptor density, determined by binding of 3H-dihydroalprenolol, was examined in the myocardium (LA = left atrium, RA = right atrium, LV1 = proximal and LV2 = distal left anterior descending artery distribution, LV3 = posterior left ventricle, S = septum, and RV = right ventricle). In control cats (no coronary occlusion or timolol) beta-receptor density of LV2 and LV3 was greater (P less than .05) than LA, RA, LV1, and RV. LV3 was greater (P less than .05) than S and RA, and LA was less than S. Longer treatment with timolol increased beta-receptor density. When compared with no timolol, beta-receptor density was greater in RA after 8 weeks and in LV1 after 2 weeks and not different in LV2 and S. Beta-receptor density and LV3 and RV were greater after 8 weeks than after 1 week or no timolol. Spearman rank correlation coefficients between dose and beta-receptor density revealed an increase (P less than .05) for all heart areas. Heart rate did not vary before timolol and was decreased after all doses of timolol. Timolol increased the mean times to coronary occlusion-induced death although the increase was not statistically significant. Timolol did not prevent postganglionic cardiac sympathetic neural discharge associated with arrhythmia. Timolol may increase beta-receptor density and decrease synaptic norepinephrine, causing a decreased release per cardiac sympathetic nerve impulse. Alternatively, molecules of timolol may accumulate in nerve endings and be released in greater concentrations at the receptors. This could explain the protection against coronary occlusion-induced arrhythmia and death.     

Cardiac and lymphocyte beta-adrenoceptors in perinephritis hypertension in the rabbit

The effects of perinephritis hypertension on lymphocyte and cardiac beta-adrenoceptors in the rabbit were examined. Hypertensive animals 8-12 weeks after surgery had an increase in cardiac weight consistent with hypertrophy compared with sham-operated age-matched controls. Specific binding of [I125] iodocyanopindolol (ICYP) to cardiac ventricular membranes was reduced in the hypertensive animals (Bmax44 +/- 14 in hypertensives and 30 +/- 15 fmoles/mg protein in controls; p less than 0.01). However, weight-matched ventricles from a group of older sham-operated normotensive rabbits showed a similar cardiac beta-receptor number to that found in the hypertensive animals. There were no changes in affinity of the ligand for the binding site. The reduction in cardiac beta-receptor density was not accompanied by changes in the chronotropic or blood pressure responses to isoprenaline in the conscious animal. Specific ICYP binding to lymphocyte beta-receptors did not differ significantly between the hypertensive and age-matched normotensive animals. Lymphocyte beta-adrenoceptors thus may not always reflect changes in heart beta-adrenoceptors, and changes in receptor density may not be directly related to blood pressure or have identifiable functional significance.     

Role of serotonergic input in the down-regulation of beta-adrenoceptors following long-term clorgyline treatment

Administration of the selective monoamine oxidase (MAO) type A-inhibiting antidepressant clorgyline (1 mg/kg per day) to rats for 21 days caused a significant decrease in cortical [3H]dihydroalprenolol binding. Selective lesioning of central serotonergic axons by 5,7-dihydroxytryptamine (5,7-DHT; confirmed by the presence of the serotonin syndrome in response to a 40 mg/kg dose of 5-hydroxytryptophan (5-HTP) or inhibition of 5-HT synthesis by parachlorophenylalanine (PCPA) caused significant 5-HT and 5-HIAA depletions in the cortex without much effect on NE and DA concentrations, but did not have any significant effect on beta-adrenoceptor density, and furthermore failed to attenuate clorgyline-induced decreases in beta-adrenoceptor density. Clorgyline treatment partially antagonized 5-HT depletion by the 5,7-DHT lesion or PCPA treatment. These findings suggest that due to their ability to raise 5-HT concentrations, MAO-inhibiting antidepressants may be a better alternative than the tricyclics in treating depressed patients with reduced 5-HT if down-regulation of beta-adrenoceptors is critical for antidepressant efficacy.     

In vivo effects of amiodarone on cardiac beta-adrenoceptor density and heart rate require thyroid hormones.

To assess if the anti-beta-adrenergic effect and the bradycardia induced by amiodarone were mediated by thyroid hormone, we investigated these effects of amiodarone in euthyroid and hypothyroid rats. We studied control rats, thyroidectomized rats, control rats treated with amiodarone (50 mg/kg for 8 days), and thyroidectomized rats treated with amiodarone. At the end of the treatment, free thyroid hormone levels (FT4 and FT3) were determined, and cardiac beta-receptor density (Bmax) and affinity (Kd) were assayed by using (-)-[125I]iodocyanopindolol as radioligand. Resting heart rate (rHR) was also assessed every day in control and thyroidectomized rats, before and after amiodarone. In hypothyroid rats, in which free thyroxine (FT4) was not detectable and free 3,5,3'-triiodothyronine (FT3) was only 16% that of euthyroid rats, Bmax (14.1 +/- 2.5 fmol/mg, n = 7) and rHR (259 +/- 9.7 beats/min, n = 6) were significantly lowered compared with euthyroid rats (Bmax:18.4 +/- 3.4 fmol/mg, n = 7; rHR:277 +/- 4.1 beats/min, n = 5). Amiodarone treatment decreased Bmax (13.6 +/- 2.9 fmol/mg, n = 8) and rHR (252 +/- 5.5 beats/min, n = 5) only in euthyroid rats and did not produce significant cardiac effects in hypothyroid rats. (Values are given as mean +/- SD.) We conclude that a minimum serum thyroid hormone concentration is a necessary condition for amiodarone to produce some of its cardiac effects. An antagonistic reaction to thyroid hormones at the cellular level can be postulated as a mechanism of the cardiac anti-beta-adrenergic action of amiodarone.     

Reversal of noradrenaline denervation-induced increase of beta-adrenoreceptor binding in rat neocortex by noradrenaline infusion

The effect of intraventricular infusion of (-)-noradrenaline (NA) on beta-receptor binding in vitro to homogenates from 6-hydroxydopamine (6-OH-DA)-denervated and from normal rat cerebral cortex was studied. NA was infused with osmotic minipumps connected to cannulas placed in the right lateral ventricle, delivering 1 or 5 microgram (-)-NA/h continuously for 9 days. One day later the rats were sacrificed and cortical tissue taken for beta-receptor (using [3H]dihydroalprenolol ([3H]DHA) as radioligand) and NA assay. The NA level in the cerebral cortex of 6-OH-DA treated rats was decreased to 70-80% of that of controls. No substantial change in the NA level was observed after infusion of 1 microgram (-)-NA/h, whereas infusion of 5 microgram/h led to a 40-60% increase compared to that of control rats infused with vehicle alone. Infusion of vehicle alone into control rats did not cause any change in [3H]DHA binding, whereas in denervated rats there was a 30-50% increase in [3H]DHA binding compared to that of controls. This increase was completely counteracted by infusion of 1 or 5 microgram (-)-NA/h. Infusion of 1 microgram (-)-NA/h to control rats did not cause any change, while infusion of 5 microgram (-)NA/h led to a significant decrease (-24%) in [3]DHA binding. The present results further support the view that the availability of NA at the receptors controls the number of beta-adrenergic receptors, thereby probably regulating the NA sensitivity of cells with beta-receptors.     

Temperature dependence of [3H]dihydroalprenolol binding in rat myocardium

The beta-adrenergic antagonist, [3H]dihydroalprenolol, was used to label binding sites in crude rat myocardial plasma membranes. The specificity of binding was dependent on the temperature of the assay. Specific binding at 22 and 37 degree C and at concentrations of radioligand less than 5 nM was consistent with binding to the myocardial beta-receptor. Binding sites labeled at 4 degree C possessed quite different properties. Binding was non-stereoselective and of lower affinity. Agonist compounds were much less effective at competing for the labeled myocardial sites at 4 degree C than at 22 degree C. Those beta-antagonists which additionally possess pharmacological "quinidine-like' activity (e.g. propranolol, alprenolol) were potent competitors at 4 degree C, but competition was non-stereoselective. In contrast atenolol, a beta-antagonist devoid of "quinidine-like' activity was ineffective at 4 degree C. Furthermore, procaine, and quinidine itself were potent competitors of [3H]dihydroalprenolol binding at 4 degree C. Thus the specificity of [3H]dihydroalprenolol binding to rat heart membranes at 4 degree C appears to be directed non-stereoselectively at that portion of the competing molecule which recognized "quinidine-like' as opposed to adrenergic activity.     

Effects of the H2-receptor agonist impromidine in human myocardium from patients with heart failure due to mitral and aortic valve disease

We determined stimulation rates of cardiac adenylate cyclase activity by isoproterenol and impromidine in particulate sarcolemmal membrane preparations from human papillary muscles resected during open heart replacement of mitral and aortic valves. In addition, specific receptor binding studies with [3H]dihydroalprenolol [( 3H]DHA) to cardiac beta-receptors and [3H]tiotidine [( 3H]TIOT) to cardiac H2-receptors were carried out in the same preparations. Compared with the response in patients with pure mitral valve stenosis, the response of cardiac adenylate cyclase activity to isoproterenol showed a marked decrease (-90%) in patients with combined mitral and aortic valve disease, corresponding to the severity of degree of insufficiency at both valves. Similar changes were observed in receptor binding studies with [3H]DHA, in which the reduction of beta-receptor density was of the same order of magnitude. In contrast, stimulation of the enzyme by impromidine and binding capacity of [3H]TIOT to cardiac H2-receptors were found to be unaltered in the same membrane preparations of all 16 patients. We conclude that treatment with H2-agonists may be a new therapeutic approach to congestive heart failure, especially in patients not responding to beta-adrenoceptor stimulation with beta-sympathomimetic drugs.     

Disposition of the diastereoisomers quinine and quinidine in the ovine fetus

The disposition of the diastereoisomers quinine and quinidine was investigated in the near-term pregnant ewe. Five sheep were administered quinine and quinidine separately in random order by a combination of bolus and 30-h iv infusion. On a subsequent occasion, four of the five sheep were also administered the two drugs simultaneously. After separate dosage, systemic clearance of quinine tended to be greater than that of quinidine (714 +/- 299 versus 422 +/- 146 mL/min, p = 0.08). Maternal renal clearance exhibited no stereoselectivity and represented less than 2% of total clearance. Simultaneous administration did not alter the disposition of either drug in the mother. After separate dosage, fetal total concentrations (Cf) of quinine and quinidine were substantially lower than maternal total concentrations, as reflected in Cf:Cm ratios of 0.15 +/- 0.06 versus 0.10 +/- 0.08, respectively. Similarly, fetal unbound concentrations (Cfu) were substantially lower than maternal unbound concentrations (Cmu; Cfu/Cmu = 0.46 +/- 0.09 for quinine and 0.23 +/- 0.09 for quinidine). This indicates the presence of fetal elimination of both isomers. Fetal renal clearances of quinine and quinidine were similar (0.34 +/- 0.24 mL/min versus 0.38 +/- 0.24 mL/min) and less than that of endogenous creatinine, indicating the absence of net renal tubular secretion. After simultaneous dosage of quinine and quinidine, Cf:Cm (0.48 +/- 0.24 and 0.31 +/- 0.19, respectively) and Cfu:Cmu (0.73 +/- 0.14 and 0.52 +/- 0.20, respectively) were greater than for separate dosages. Fetal renal clearance of both drugs was unchanged, suggesting that the higher Cfu:Cmu ratios after simultaneous dosage were due to mutual inhibition of the fetal metabolism of these drugs.     

去甲肾上腺素和多巴酚丁胺联用与多巴胺单用对感染性休克绵羊内脏灌注的比较

AIM: To compare the effect of norepinephrine-dobutamine with dopamine alone on splanchnic perfusion in sheep with septic shock. METHODS: Twenty sheep with septic shock induced by lipopolysaccharides were divided into two groups. When systolic pressure decreased by 5.3 kPa, basic values of hemodynamic parameters and intestinal intramucosal pH (pHi) were recorded. Each group was randomized to receive an intravenous infusion of norepinephrine-dobutamine or dopamine, and titrated to obtain mean arterial pressure (MAP) > 12 kPa with an optimal cardiac preload. Hemodynamic parameters and mucosal pHi were repeated at 1, 2, 3, and 4 h after basic measurement. RESULTS: After norepinephrine-dobutamine or dopamine infusion, MAP, cardiac output, and oxygen delivery increased in all animals compared with basic values in both groups (P < 0.05). Compared with baseline values, lactate concentrations decreased at 3 h and 4 h [from (4 +/- 2) mmol/L to (2 +/- 1) mmol/L] in the norepinephrine-dobutamine group (P < 0.05 ). Arterial lactate concentrations had no change in dopamine group, but arterial pH decreased from 7.40 +/- 0.05 to 7.26 +/- 0.06 at 1 h (P < 0.05 ). No difference in pHi was found in dopamine group, but in the norepinephrine-dobutamine group, compared with baseline, pHi increased from 7.19 +/- 0.04 to 7.36 +/- 0.07 at 3 h (P < 0.05). CONCLUSION: Both norepinephrine-dobutamine and dopamine alone could improve systemic hemodynamics in sheep with septic shock, but norepinephrine-dobutamine was better than dopamine on splanchnic perfusion.     

A pharmacokinetic study of diphenhydramine transport across the blood-brain barrier in adult sheep: potential involvement of a carrier-mediated mechanism.

The purpose of this study was to examine the disposition of diphenhydramine (DPHM) across the ovine blood-brain barrier (BBB). In six adult sheep, we characterized the central nervous system (CNS) pharmacokinetics of DPHM in brain extracellular fluid (ECF) and cerebrospinal fluid (CSF) using microdialysis in two experiments. In the first experiment, DPHM was administered via a five-step i.v. infusion (1.5, 5.5, 9.5, 13.5, and 17.5 microg/kg/min; 7 h per step). Average steady-state CNS/total plasma concentration ratios (i.e., [CNS]/[total plasma]) for steps 1 to 5 ranged from 0.4 to 0.5. However, average steady-state [CNS]/[free plasma] ratios ranged from 2 to 3, suggesting active transport of DPHM into the CNS. Plasma protein binding averaged 86.1 +/- 2.3% (mean +/- S.D.) and was not altered with increasing drug dose. Plasma, CSF, and ECF demonstrated biexponential pharmacokinetics with terminal elimination half-lives (t1/2beta) of 10.8 +/- 5.4, 3.6 +/- 1.0, and 5.3 +/- 4.2 h, respectively. The bulk flow of CSF and transport-mediated efflux of DPHM may explain the observed higher CNS clearances. In the second experiment, DPHM was coadministered with propranolol (PRN) to examine its effect on blood-brain CSF and blood-brain ECF DPHM relationships. Plasma total DPHM concentration decreased by 12.8 +/- 6.3% during PRN, whereas ECF and CSF concentrations increased (88.1 +/- 45.4 and 91.6 +/- 34.3%, respectively). This increase may be due to the inhibitory effect of PRN on a transporter-mediated efflux mechanism for DPHM brain elimination.