Ultradian variations in sensitivity of rat aorta rings to noradrenaline

Vascular smooth muscle desensitization in rabbit epigastric and mesenteric arteries during hemorrhagic shock

The decompensatory phase of hemorrhage (shock) is caused by a poorly defined phenomenon termed vascular hyporeactivity (VHR). VHR may reflect an acute in vivo imbalance in levels of contractile and relaxant stimuli favoring net vascular smooth muscle (VSM) relaxation. Alternatively, VHR may be caused by intrinsic VSM desensitization of contraction resulting from prior exposure to high levels of stimuli that temporarily adjusts cell signaling systems. Net relaxation, but not desensitization, would be expected to resolve rapidly in an artery segment removed from the in vivo shock environment and examined in vitro in a fresh solution. Our aim was to 1) induce shock in rabbits and apply an in vitro mechanical analysis on muscular arteries isolated pre- and postshock to determine whether VHR involves intrinsic VSM desensitization, and 2) identify whether net VSM relaxation induced by nitric oxide and cyclic nucleotide-dependent protein kinase activation in vitro can be sustained for some time after relaxant stimulus washout. The potencies of phenylephrine- and histamine-induced contractions in in vitro epigastric artery removed from rabbits posthemorrhage were decreased by ∼0.3 log units compared with the control contralateral epigastric artery removed prehemorrhage. Moreover, a decrease in KCl-induced tonic, relative to phasic, tension of in vitro mesenteric artery correlated with the degree of shock severity as assessed by rates of lactate and K(+) accumulation. VSM desensitization was also caused by tyramine in vivo and PE in vitro, but not by relaxant agents in vitro. Together, these results support the hypothesis that VHR during hemorrhagic decompensation involves contractile stimulus-induced long-lasting, intrinsic VSM desensitization.

Smooth-muscle BMAL1 participates in blood pressure circadian rhythm regulation

As the central pacemaker, the suprachiasmatic nucleus (SCN) has long been considered the primary regulator of blood pressure circadian rhythm; however, this dogma has been challenged by the discovery that each of the clock genes present in the SCN is also expressed and functions in peripheral tissues. The involvement and contribution of these peripheral clock genes in the circadian rhythm of blood pressure remains uncertain. Here, we demonstrate that selective deletion of the circadian clock transcriptional activator aryl hydrocarbon receptor nuclear translocator-like (Bmal1) from smooth muscle, but not from cardiomyocytes, compromised blood pressure circadian rhythm and decreased blood pressure without affecting SCN-controlled locomotor activity in murine models. In mesenteric arteries, BMAL1 bound to the promoter of and activated the transcription of Rho-kinase 2 (Rock2), and Bmal1 deletion abolished the time-of-day variations in response to agonist-induced vasoconstriction, myosin phosphorylation, and ROCK2 activation. Together, these data indicate that peripheral inputs contribute to the daily control of vasoconstriction and blood pressure and suggest that clock gene expression outside of the SCN should be further evaluated to elucidate pathogenic mechanisms of diseases involving blood pressure circadian rhythm disruption.

Altered clock gene expression and vascular smooth muscle diurnal contractile variations in type 2 diabetic db/db mice

This study was designed to determine whether the 24-h rhythms of clock gene expression and vascular smooth muscle (VSM) contractile responses are altered in type 2 diabetic db/db mice. Control and db/db mice were euthanized at 6-h intervals throughout the day. The aorta, mesenteric arteries, heart, kidney, and brain were isolated. Clock and target gene mRNA levels were determined by either real-time PCR or in situ hybridization. Isometric contractions were measured in isolated aortic helical strips, and pressor responses to an intravenous injection of vasoconstrictors were determined in vivo using radiotelemetry. We found that the 24-h mRNA rhythms of the following genes were suppressed in db/db mice compared with control mice: the clock genes period homolog 1/2 (Per1/2) and cryptochrome 1/2 (Cry1/2) and their target genes D site albumin promoter-binding protein (Dbp) and peroxisome proliferator-activated receptor-γ (Pparg) in the aorta and mesenteric arteries; Dbp in the heart; Per1, nuclear receptor subfamily 1, group D, member 1 (Rev-erba), and Dbp in the kidney; and Per1 in the suprachiasmatic nucleus. The 24-h contractile variations in response to phenylephrine (α(1)-agonist), ANG II, and high K(+) were significantly altered in the aortas from db/db mice compared with control mice. The diurnal variations of the in vivo pressor responses to phenylephrine and ANG II were lost in db/db mice. Moreover, the 24-h mRNA rhythms of the contraction-related proteins Rho kinase 1/2, PKC-potentiated phosphatase inhibitory protein of 17 kDa, calponin-3, tropomyosin-1/2, and smooth muscle protein 22-α were suppressed in db/db mice compared with control mice. Together, our data demonstrated that the 24-h rhythms of clock gene mRNA, mRNA levels of several contraction-related proteins, and VSM contraction were disrupted in db/db mice, which may contribute to the disruption of their blood pressure circadian rhythm.

The effect of experimental diabetes on the twenty-four-hour pattern of the vasodilator responses to acetylcholine and isoprenaline in the rat aorta

The aim of this study was to investigate whether time-dependent variations in the relaxant effect of acetylcholine, an endothelium-dependent vasorelaxant via muscarinic receptors, and isoprenaline, a nonselective beta-adrenoceptor agonist in rat aorta, are influenced by streptozotocin (STZ)-induced experimental diabetes. Adult male rats were divided randomly into two groups: control and STZ-induced (STZ, 55 mg/kg, intraperitoneal) diabetes. The animals were synchronized to a 12:12 h light-dark cycle (lights on 08:00 h) and sacrificed at six different times of day (1, 5, 9, 13, 17, and 21 hours after lights on; HALO) eight weeks after STZ injection. The in vitro responsiveness of thoracic aorta rings obtained from control and diabetic rats to acetylcholine (10(-9)-10(-5) M) and isoprenaline (10(-10)-10(-3) M) was determined in six different times. EC(50) (the concentration inducing half of the maximum response) values and maximum responses were calculated from cumulative concentration-response curves of the agonists and were analyzed with respect to time and STZ treatment. Treatment, time, and interactions between treatment and time were tested by two-way analysis of variance (ANOVA). To analyze differences due to biological time, one-way ANOVA was used. STZ treatment did not significantly change EC(50) values or maximum responses for both agonists. There were statistically significant time-dependent variations in the EC(50) values for isoprenaline and maximum responses for both acetylcholine and isoprenaline in control groups by one-way ANOVA, but significant time-dependent variations disappeared in the aortas isolated from STZ-induced diabetic rats. The vasodilator responses to acetylcholine and isoprenaline failed to show any significant interaction (treatmentxtime of study) between STZ treatment and time of sacrifice in both EC(50) values and maximum responses by two-way ANOVA. These results indicate there is a basic temporal pattern in the responses to acetylcholine and isoprenaline in rat aorta which continues in diabetes. It is shown for the first time that experimental diabetes does not change the 24 h pattern of responses to acetylcholine and isoprenaline, and that time-dependent variations in the responses to these agonists disappear in diabetic animals. Although further studies are required to define the underlying mechanism(s) of these findings, results suggest that experimental diabetes can modify the time-dependent vasorelaxant responses of rat aorta. This may help to understand the circadian rhythms in cardiovascular physiology and pathology or in drug effects in diabetes.

Reduced alpha-adrenoceptor responsiveness and enhanced baroreflex sensitivity in Cry-deficient mice lacking a biological clock

To reveal the role of clock genes in generating the circadian rhythm of baroreflexes, we continuously measured mean arterial pressure and baroreflex sensitivity in free-moving normal wild-type mice, and in Cry-deficient mice which lack a circadian rhythm, in constant darkness for 24 h. In wild-type mice the mean arterial pressure was higher at night than during the day, and was accompanied by a significantly enhanced baroreflex sensitivity of -13.6 +/- 0.8 at night compared with -9.7 +/- 0.7 beats min(-1) mmHg(-1) during the day (P < 0.001). On the other hand, diurnal changes in arterial pressure disappeared in Cry-deficient mice with remarkably enhanced baroreflex sensitivity compared with wild-type mice (P < 0.001): -21.9 +/- 1.6 at night and -23.1 +/- 2.1 beats min(-1) mmHg(-1) during the day. Moreover, the mean arterial pressure response to 10 microg kg(-1) of phenylephrine, an alpha1-adrenoceptor agonist, was severely suppressed in Cry-deficient mice regardless of time, while that for the wild-type mice was 10.1 +/- 1.9 mmHg in the night, significantly lower than 22.0 +/- 3.5 mmHg in the day (P < 0.01). These results suggest that CRY genes are involved in generating the circadian rhythm of baroreflex sensitivity, partially by regulating alpha(1)-adrenoceptor-mediated vasoconstriction in peripheral vessels.

Biological-time-dependent differences in effect of verapamil on rat aorta and influence of endothelium

The biological-time-dependent variation in the vasodilator effect of verapamil on rat thoracic aorta was assessed in both endothelium-intact and denuded preparations. Groups of adult male rats were housed in light from 08:00 to 20:00 and in darkness from 20:00 to 08:00 and sacrificed at six different times of the day (1, 5, 9, 13, 17, and 21 hours after lights on; HALO). Verapamil caused concentration-dependent relaxations in both endothelium-intact and denuded aortic rings precontracted with phenylephrine (Phe). In endothelium-intact rings, neither the AUC nor the EC50 values for verapamil exhibited significant biological-time-dependent effects, as determined by one-way analysis of variance (ANOVA). In endothelium-denuded rings, AUC values did vary in a statistically significant manner according to the biological time of study, while the EC50 values did not. Endothelium denudation led to an increase in EC50 values at almost every time point. Statistically significant interactions between the biological time of study and treatment (intact vs. denuded endothelium) in both AUC and EC50 values were documented by two-way ANOVA; this indicated differences in the clock-time staging of verapamil-induced relaxation in endothelium-denuded versus intact aortic rings.

In vitro susceptibility rhythms. II. Biological-time-dependent differences in effect of beta 1- and beta 2-adrenergic agonists of rat aorta and influence of endothelium

Time-dependent variations in the vasodilator effects of beta-adrenergic agonists terbutaline (Ter) and dobutamine (Dob) were studied in isolated rings of rat thoracic aorta in both endothelium-intact and endothelium-denuded preparations. Rats were housed in light from 08:00 to 20:00 and in darkness from 20:00 to 08:00 and sacrificed at six different times of the day. In endothelium-intact and endothelium-denuded aortic rings precontracted submaximally with phenylephrine (Phe), addition of Ter and Dob produced concentration-dependent relaxations. Removal of endothelium reduced the relaxant responses and area under curve (AUC) values and augmented the EC50 values to Ter and Dob at most, but not all, time points. Two-way analysis of variance (ANOVA) revealed that AUCs, maximum responses, and EC50 values significantly depended on both treatment (endothelium intact/endothelium denuded) and time of sacrifice. Results of the present study clearly show that in vitro sensitivity of rat thoracic aorta to beta-adrenergic agonists displays temporal variations depending on the time of animal sacrifice, and the presence of endothelium modifies the rhythmicity in beta-adrenergic activity. These variations may be due to the circadian rhythmicity in the adenylyl cyclase-cAMP-phosphodiesterase system that mediates the responses to beta-adrenergic agonists.

In vitro evidence of tissue susceptibility rhythms. I. Temporal variation in effect of potassium chloride and phenylephrine on rat aorta

In this study, time-dependent variations in the in vitro sensitivity of rat thoracic aorta rings to potassium chloride (KCl) and phenylephrine (Phe) were investigated. Animals were synchronized with a 12h light and 12h darkness (lights on 08:00-20:00) schedule, and thoracic aortas were obtained at six different times of the day (1, 5, 9, 13, 17, and 21 hours after lights on). In order to avoid endothelial influence, all experiments were performed in endothelium-denuded preparations. Responses to KCI showed time-dependent variations in all the concentrations used. Phe-induced contractions also exhibited time-dependent differences. The rhythmic pattern of Phe responses did not change with the presence of the alpha1-adrenergic antagonist prazosin. In addition, both the EC50 values of KCl and Phe, and also the K(B) values of prazosin, displayed rhythmicity. In conclusion, time of obtaining tissues is an important factor for experimental standardization in, at least, vascular smooth muscle preparations.

Twenty-four-hour variations in the effect of nitrodilators in rat aorta: lack of influence of the endothelium

Time-dependent variations of the vasodilator effects of sodium nitroprusside and glyceryl trinitrate on isolated smooth muscle have been studied on rings of rat thoracic aorta, both endothelium-intact and endothelium-denuded. For most of the concentrations of sodium nitroprusside used the induced relaxations were significantly dependent on the time the tissues were obtained. However, significant temporal differences were obtained for glyceryl trinitrate-induced relaxations at lower concentrations only for both endothelium-intact and endothelium-denuded preparations. EC50 values of sodium nitroprusside and glyceryl trinitrate (i.e. the concentrations inducing half the maximum response) were also significantly different and they had quite similar rhythmic features both in endothelium-intact and in endothelium-denuded preparations. These results clearly show that the in-vitro sensitivity of rat thoracic aorta to nitrodilator agents varies over a 24-h period and thus depends on when the animals were killed; the presence of endothelium does not change the rhythm of nitrodilator activity. These variations might be a result of circadian rhythm in the guanylate cyclase-cGMP system which mediates responses to nitrodilator agents.

Twenty-four-hour variations in the sensitivity of rat aorta to vasoactive agents

The presence of time-dependent variations in the in vitro sensitivity of aorta preparations to either vasoconstricting or relaxing agents was investigated in rats maintained in light for 08:00 to 20:00 and in darkness from 20:00 to 08:00. Rat thoracic aorta rings were obtained from animals sacrificed at four different times of the day. The rat aorta was found to be sensitive to the constricting effect of phenylephrine at 15:00, and of 5-hydroxytryptamine at 21:00. On the other hand, both endothelium-dependent and -independent relaxations were more remarkable at 03:00 than at other times of the day. These variations represented significant circadian rhythms when analyzed by analysis of variance. Different in vitro responsiveness to these agents might reflect changes in the sensitivity and/or number of related receptors in vascular preparations. In conclusion, the circadian time of animal sacrifice to obtain vascular preparations constitutes an important aspect of the research method and a key determinant of findings.

Signal transduction in animal models of normotension and hypertension

Experiments in inbred strains of normotensive and hypertensive rats have clearly demonstrated circadian rhythms in blood pressure and heart rate. Pre- and postsynaptic signal transduction processes in vitro can, but need not, vary with circadian time, greatly depending on the strain of rats investigated. These data highlight the notion of a strain-dependent, and thus genetic, regulation of the cardiovascular system. Obviously, circadian rhythms in blood pressure cannot be explained by single biochemical parameters, but results from both in vitro and in vivo studies give first evidence that the vascular nitric oxide-cGMP system may be involved in the circadian regulation of blood pressure in WKY and SHR rats. In secondary hypertensive TGR and in their normotensive controls, SPRD, the guanylyl cyclase system does not seem to play a role in circadian blood pressure regulation. In neither of the four strains studied did aortic adenylyl cyclase show any time-dependent variation. Because vascular tissue was taken from the thoracic aorta of the rats, a contribution of adenylyl cyclase to circadian blood pressure regulation in small resistance arteries cannot be ruled out. Further studies in different parts of the vascular tree are needed to definitely answer that question. No data are available on time-dependent variation in the activity of phospholipase C, the second messenger pathway of vascular alpha-adrenoceptors and angiotensin II AT1-receptors, both of which mediate vasoconstriction. Future research into this system will be helpful in identifying mechanisms involved in blood pressure regulation in SPRD and TGR.

Effect of age on noradrenaline responses in rat tail artery and aorta: role of endothelium

1. We have analysed the impact of ageing on the contractile responses induced by noradrenaline on endothelium intact and denuded aorta and tail artery rings from Sprague-Dawley rats. In addition, the influence of age on noradrenaline stimulation of phosphoinositide hydrolysis was investigated. 2. The sensitivity and the phosphatidylinositol hydrolysis to noradrenaline in aorta and tail artery were not modified by age. Intact tail artery rings showed a greater maximal contraction (Emax) to noradrenaline in old as compared to young animals. However, no Emax modification by age was observed in aorta (intact or denuded) and in denuded tail artery rings. 3. Removal of endothelial cells resulted in an increase of noradrenaline sensitivity but not the Emax in aorta from each age group. 4. In contrast, the absence of endothelium did not modify (young rats) or diminish (aged rats) the alpha 1-adrenoceptor-mediated responses in tail artery. 5. These results seem to indicate that: (1) there is no influence of age on noradrenaline responses in presence of endothelium; and (2) responses in denuded preparations seem to indicate a differential role of endothelium on noradrenaline responses obtained in different vascular beds.