Can We Apply Results From Large to Small Arteries?
To the Editor:
The study of Sausbier et al,1 published in a recent issue of Circulation Research, investigated the mechanism of nitric oxide (NO)/cGMP-dependent vasorelaxation. For this purpose, the authors carried out a series of experiments using both large and small resistance arteries from wild-type and cGMP kinase I-deficient (cGKI−/−) mice. The experiments were performed using two different techniques, the wire-based isometric tension technique (wire myograph) and the pressure-perfusion technique (arteriograph). The aim of the present commentary is to point out some methodological issues that, we believe, should be taken into account when extrapolating results from large arteries onto small resistance arteries: (1) Differences in anatomic structure of large and resistance arteries.2 (2) Differences in the significance of contractile and relaxant function of large and resistance arteries. (3) Differences in the mechanisms of myogenic and agonist-induced precontractile responses and in the mediators of endothelium-dependent relaxation in large and resistance arteries. (4) Differences in the responsiveness of wire-mounted (isometric) and pressurized (isobaric) arterial preparations.
To study the relaxant responses, resistance arteries were precontracted by changing the intraluminal pressure (myogenic activation), whereas large arteries were precontracted by norepinephrine (α-adrenergic activation). The different activation also refers to the marked differences in the primary function of large and resistance arteries. Resistance arteries are indeed continuously subjected to changes in mechanical forces (flow and pressure) that regulate active vasomotion, fitting blood flow continuously to local demands. The primary function of large arteries is, however, different, and it is rather to damp the energy generated by the blood ejected by the heart at each systole. In other words, the contractile and dilatory function of large arteries is fairly limited and therefore is not comparable with that of resistance arteries.
Moreover, the mechanisms involved in myogenic activity/tone are different from the mechanisms involved in the contractile response to norepinephrine. Most importantly, myogenic tone needs only low intracellular calcium levels, which is not the case when regarding the contraction to agonists.3 Similar functional differences can be pointed out in endothelium-dependent relaxant responses. Endothelium-dependent relaxation in resistance arteries involves NO, endothelium-derived hyperpolarizing factor (EDHF), as well as prostanoids, which show a complex interaction in the determination of the basal tone.4,5⇓ In contrast, several studies found that the endothelium-dependent relaxation in most large arteries involves only NO.6
Regarding the two different experimental techniques used in the study, it is worth noting that the responsiveness of arteries to agonists shows marked differences when studied as wire-mounted or cannulated preparations, but also when compared under isometric and isobaric conditions.7,8⇓ When comparing the two different techniques, it was previously found that much higher agonist concentrations are required to evoke tension changes in wire-mounted rings than to induce diameter changes in cannulated segments.7–9⇓⇓ However, when using the same technique, isometric conditions seem to be accompanied with enhanced responsiveness compared with that under isobaric conditions.8
The differences in the responsiveness to endothelium-dependent relaxants are also illustrated by some of the results of the study. Most studies on resistance as well as on large arteries demonstrate that 1 μmol/L acetylcholine (Ach) induces maximal relaxation and therefore can be considered as a high concentration.6 The authors, however, considered 1 μmol/L Ach as a low and 10 μmol/L Ach as a high concentration. Ach at 1 μmol/L induced maximal relaxation in resistance arteries but induced only ≈40% relaxation in large-artery preparations (see Figure 2 of Sausbier et al1). There were apparently differences in the mechanisms as well, because the inhibitory effect of iberiotoxin was still present in large arteries at 10 μmol/L Ach, whereas it nearly vanished in the resistance artery already at 1 μmol/L Ach. The same applies to the results obtained with 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA-NO). DEA-NO at 10 μmol/L evoked maximal relaxation in the large artery, but only partial relaxation was reached in the resistance arteries from cGKI−/− mice. These differences also argue for the considerable functional differences that exist between large and small resistance arteries. We believe that it could be useful to measure NO release under Ach-induced relaxations and use the determined concentrations as a basis for comparison with DEA-NO-induced responses.
In summary, the comments presented herein are intended to point out some of the methodological issues that might need to be taken into account when drawing the conclusions of the study. We believe that the results of this study1 are worth being revisited in a future investigation that uses only resistance artery preparations tested by the arteriograph.
- ↵Sausbier M, Schubert R, Voigt V, Hirneiss C, Pfeifer A, Korth M, Kleppisch T, Ruth P, Hofmann F. Mechanisms of NO/cGMP-dependent vasorelaxation. Circ Res. 2000; 87: 825–830.
- ↵Rhodin JAG. Architecture of the vessel wall.In: Bohr DF, Somlyo AP, Sparks HV Jr, eds. Handbook of Physiology, Section 2: The Cardiovascular System, Vol II. Bethesda, Md: American Physiology Society; 1980: 1–31.
- ↵Nishikawa Y, Stepp DW, Chilian WM. Nitric oxide exerts feedback inhibition on EDHF-induced coronary arteriolar dilation in vivo. Am J Physiol. 2000; 279: H459–H465.
- ↵Scotland RS, Chauhan S, Vallance PJT, Ahluwalia A. An endothelium-derived hyperpolarizing factor-like factor modulates myogenic constriction of mesenteric resistance arteries in the absence of endothelial nitric oxide synthase-derived nitric oxide. Hypertension. 2001; 38: 833–839.
- ↵Iglarz M, Matrougui K, Lévy BI, Henrion D. Chronic blockade of endothelin ETA receptors improves flow dependent dilation in resistance arteries of hypertensive rats. Cardiovasc Res. 1998; 39: 657–664.