Adrenoceptor and cholinoceptor modulation of rat pulmonary vein cardiac muscle contractility.

Cardiac muscle extends into mammalian pulmonary veins for variable distances according to species. This study has addressed the autonomic control of electrically paced cardiac muscle of the pulmonary vein of the rat. Contractile responses of Wistar rat pulmonary veins were investigated under isometric conditions in vitro. Vessels were electrically paced at 1 Hz (10 V, 1 ms pulse width). Acetylcholine (ACh, 1 nM-10 microM) attenuated the contractile response (maximum inhibition at 1 microM, 41+/-15%, mean+/-SD). The attenuation was inhibited by atropine (p<0.05) and partially inhibited (7+/-4%, mean+/-SD, p<0.01) by removal of the endothelium. Noradrenaline (NA, 1 nM-10 microM) augmented the cardiac muscle contractility in a fashion partially inhibited by atenolol; augmentation at 10 microM was reduced from 169+/-9% (n=6) to 135+/-9% (n=5), (p<0.05). The ability of ACh to attenuate the contractile responses was unaffected by the presence of NA. In conclusion, ACh has a muscarinic receptor-mediated negative inotropic effect upon the cardiac muscle of the pulmonary vein of the rat mediated in part by the endothelium. The cardiac muscle expresses a positive inotropic response to NA partly mediated by beta1-adrenoceptors that can be antagonised by ACh. Therefore, pulmonary vein cardiac muscle function is modulated by competing autonomic influences which may be of significance to the generation of atrial fibrillation events.

Implications for repetitive application of acetylcholine in the determination of the mechanisms of endothelium-dependent relaxation.

The influence of repetitive vasodilator concentration response curve determination was investigated in rat mesenteric and femoral small arteries. Arteries were precontracted with noradrenaline and relaxed with acetylcholine (ACh) or K+. Mesenteric arteries exhibited attenuation of ACh-stimulated relaxations during repetitive precontraction/relaxation cycles that was not prevented by SQ29548. Apamin, but not charybdotoxin, prevented the attenuation of this relaxation response. Borderline (p=0.064) statistical differences in the relaxations of mesenteric arteries in response to ACh remained in the presence of the nitric oxide synthase inhibitor L-NNA. In contrast, femoral arterial relaxations increased with repeated acetylcholine challenges. The enhanced responsiveness was prevented with L-NNA or Ba2+. In one experimental series, Ba2+ appeared to be without influence upon K+-stimulated relaxation of femoral arteries but a significant inhibitory effect was revealed when appropriate time control experiments were considered. These experiments reveal that impairment of SK(Ca) function and, to a lesser extent, an impaired NO signalling account for the attenuation of the relaxation responses of rat mesenteric arteries. In contrast, alterations of nitric oxide signalling and inward rectifier K channel activity contribute to enhanced relaxation responses in rat femoral arteries. These experiments highlight the importance of appropriate time control experiments for the proper interpretation of results derived from pharmacological experiments directed toward the elucidation of mechanisms of arterial vasorelaxation.

Influence of experimental reduction of arterial media : lumen ratio on agonist-stimulated contractions in hypertension.

OBJECTIVE: This study investigated the relationship between the arterial media thickness : lumen diameter (M : L) ratio and arterial contractile responses in the spontaneously hypertensive rat (SHR) and its Wistar-Kyoto control (WKY) under isobaric conditions. METHODS: Femoral arteries with an experimentally reduced M : L ratio were produced in one hindlimb of both rat strains, by partial ligation of the ipsilateral iliac artery. Arterial structure and contractile responses were assessed in an arteriograph. Contractile responses of these vessels to the vasoconstrictor agonists noradrenaline and phenylephrine were determined after generation of spontaneous myogenic tone and compared with those from contralateral hindlimbs determined at both estimated in-vivo pressures and at 80 mmHg. Control SHR and WKY arterial responses were also compared. RESULTS: Under both pressure conditions, relaxed M : L ratios were significantly greater in SHR than in WKY arteries and were also significantly reduced in arteries distal to the ligature in both strains. In no comparison was a greater M : L associated with a greater agonist-stimulated contractile response. However, increased M : L were associated with narrower lumens across the concentration response relationships under some, but not all, conditions. CONCLUSION: An increased arterial M : L ratio does not effect an exaggerated contractile function in myogenically active resistance arteries in vitro. However, narrower lumen diameters associated with the increased M : L would provide a means for increased vascular resistance. These observations support the hypothesis that an increased M : L does not provide a means for a contractile response amplifier but may provide a means for a vascular resistance amplifier under some circumstances.

Functional evidence that K+ is the non-nitric oxide, non-prostanoid endothelium-derived relaxing factor in rat femoral arteries.

The mechanisms of K(+)-induced relaxation and of acetylcholine (ACh)-stimulated, endothelium-dependent relaxation were assessed in rat femoral arteries mounted in a myograph. ACh-stimulated (1 nM-1 microM) relaxation of arteries precontracted with 1 microM noradrenaline was mostly resistant to the combination of indomethacin (INDO; 10 microM) and N(omega)-nitro-L-arginine (L-NNA, 100 microM). The remaining relaxation was abolished by 30 mM K(+) or ouabain (1 mM) and significantly reduced by 30 microM Ba(2+) or charybdotoxin (ChTx; 100 nM) plus apamin (100 nM). K(+)-induced relaxation effected by raising [K(+)](o) by 0.5-4 mM was endothelium-independent and inhibited by ouabain and Ba(2+). These results indicate that ACh-stimulated relaxations are effected mainly by a non-prostanoid, non-nitric oxide mechanism, presumably an endothelium-derived hyperpolarising factor (EDHF). Relaxations stimulated by EDHF and K(+) are both mediated by Na(+)-K(+) ATPase and inward rectifier potassium channels (K(IR)). This study provides further functional evidence that EDHF is K(+) derived from endothelial cells that relaxes arterial smooth muscle subsequent to activation of Na(+)-K(+) ATPase and K(IR).

The inhibition of ureteral motility by periureteral adipose tissue.

Perivascular adipose tissue exerts an anticontractile influence on vascular smooth muscle. This study was conducted to determine whether periureteral adipose tissue (PUAT) could exert a similar influence upon ureteral smooth muscle. Acetylcholine-stimulated (10(-7) M-10(-4) M) contractile responses of ureteral segments obtained from male Wistar rats were recorded in the presence and absence of PUAT. Ureters with PUAT generated phasic contractile responses with significantly lower frequencies (P < 0.001) and magnitudes (P < 0.001) compared with ureters cleared of their periureteral adipose tissue. Removal of PUAT significantly increased the frequency (P < 0.01) and magnitude (P < 0.01) of the contractile responses. Bioassay experiments demonstrated that ureters with PUAT released a transferable factor that significantly reduced frequencies (P < 0.05), but not magnitudes, of the contractile responses of ureters cleared of PUAT. The nitric oxide synthase inhibitor L-NNA (10(-4) M) did not significantly influence the anticontractile effect exerted by ureters with PUAT. This is the first study to demonstrate that ureteral motility is influenced by its surrounding adipose tissue. The PUAT has an anticontractile effect which is mediated by a transferable factor released from the PUAT. The identity of the factor is unknown but does not exert its effect through nitric oxide.

Arterial myogenic properties of the spontaneously hypertensive rat.

When subject to a transmural pressure gradient resistance arteries develop a spontaneous, intrinsically initiated contraction which varies according to the pressure stimulus and occurs in the absence of vasoconstrictor agonists. Such pressure-dependent active changes in vascular tone are indicative of the vascular myogenic response and contribute to autoregulation and the setting of total peripheral resistance and hence blood pressure regulation. The myogenic behaviour of blood vessels provides the background tone upon which other vasomotor influences act. Hypertension is associated with a raised vascular resistance and in this article the evidence for increased myogenic activity contributing to the raised vascular resistance is reviewed. Although there are some cases that provide evidence for exaggerated myogenic responsiveness in resistance arteries taken from hypertensive animals it is not possible to conclude that enhanced myogenic contractile responses within normal pressure ranges contribute to the raised total peripheral resistance. However, the myogenic tone of the resistance arteries of the various vascular beds is subject to differing modulatory influences in hypertensive animals and their normotensive controls which may contribute to the aetiology of hypertension.

Influence of experimental reduction of media/lumen ratio on arterial myogenic properties of spontaneously hypertensive and Wistar-Kyoto rats.

In the present study, myogenic properties of femoral arteries from control hindlimbs and those distal to external iliac artery partial ligation of spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto (WKY) rats were assessed. Arterial pressure was reduced distal to the ligature in both strains. Media thickness/lumen diameter (M/L) ratios of control (unligatured) SHRs were greater than in unligatured WKY rats and were reduced in arteries distal to the ligature (ligatured) within each strain. In none of the comparisons was a greater M/L ratio associated with greater maximal myogenic contractions, but increased M/L ratios were associated with a shift of myogenic activity to a higher pressure range in all comparisons. SHR ligatured arteries produced greater pressure-dependent contractile responses than WKY rat unligatured arteries, although arterial structures were not significantly different. Wall stress was similar in all arteries within the 60-120 mmHg pressure range with myogenic tone in spite of large differences in arterial structure. The utilization of arteries with experimentally altered structure provides further evidence that increased M/L ratios are not associated with greater peak pressure-dependent contractile responses and that arterial wall stress is maintained within a narrow range through an interaction between arterial wall geometry and smooth muscle contractile function.

Arterial structural changes in hypertension: a consideration of methodology, terminology and functional consequence.

Alteration of resistance artery geometric design is a hallmark of established hypertension. In particular, there is an increased media thickness: lumen diameter ratio. In this review, consideration is given to the methods available for assessment of vascular structure, the terminology used to describe vascular structural changes in hypertension and the possible functional consequences of altered arterial design in the maintenance of the hypertensive state. In light of these considerations, a number of proposals are made concerning quantification strategies and descriptor terminologies. In particular, a simple descriptor terminology is proposed that describes such structural differences but makes no assumptions concerning the underlying processes that account for altered arterial structure in hypertension.