In vitro Analysis of Alpha-Adrenoceptor Interactions with the Myogenic Response in Resistance Vessels

Abstract

The interaction of alpha 1- and alpha 2-adrenoceptor constriction of isolated rat cremaster small arteries (mean diameter +/- SEM, 114 +/- 5 microns) with the myogenic mechanism was examined with in vitro videomicroscopy. Microdissected vessels were double-cannulated with glass micropipets in a tissue bath, and intraluminal pressure was set at 65 mm Hg baseline pressure. Norepinephrine (NE) concentration-response curves were obtained alone and in the presence of prazosin or rauwolscine to establish the concentration of NE required to selectively stimulate alpha 1- or alpha 2-adrenoceptors. A bimodal response was produced by NE alone, indicating interaction with more than one receptor population. Rauwolscine and prazosin each produced dextral displacement at low (< 0.1 microM) NE concentrations. However, while prazosin exhibited competitive antagonism, rauwolscine did not antagonize NE at concentrations > or = EC50 value. Thus, both alpha 1- and alpha 2-adrenoceptors mediate constriction at low NE concentrations, but only alpha 1-receptors contributed to constriction at intermediate and high concentrations. These data are in contrast to previous findings for the same vessels studied in vivo. Diameter responses to increases and decreases in transmural pressure from baseline were examined in the relaxed (passive) state with nitroprusside present, during spontaneous intrinsic tone, and during induced alpha 1- or alpha 2-tone (EC20 concentration of NE plus rauwolscine or prazosin). Myogenic constriction during increases in lumen pressure and myogenic inhibition of tone during decreases in pressure were greatest during alpha 2-tone, less during intrinsic tone and least during alpha 1-tone.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic characteristics and underlying mechanisms of renal blood flow autoregulation in the conscious dog

“…The observed response time in the range of 30 s is very similar to those observed by other authors in visualized arterioles in the hindlimb muscle of cats (25) and dogs (16,18) as well as in the cremaster muscle of rats (22,36). The direct comparison in the present study of the vascular responses in the renal and the hindlimb circulation in animals of the same strain (in 3 of them, also in the same individuals) clearly shows that there is a distinct difference in the time course of the myogenic response in these two vascular beds.…”

The myogenic response: established facts and attractive hypotheses

“…Concerning the myogenic response in vitro, it was observed that phenylephrine, an α " -adrenergic agonist, produced an augmentation of the myogenic response in rat femoral arteries [81] and in rat mesenteric arteries [76]. The degree of augmentation by α-adrenergic agonists was shown to depend on the type of α-receptor involved [86]. However, the non-selective α-adrenoceptor agonist noradrenaline did not change the myogenic response in dog renal arteries [38].…”

Renal Autoregulation in Health and Disease

“…The myogenic response of renal afferent arterioles of mice, rats, and dogs (internal ID: 8 -20 m) is often complete within 5-10 s, whereas in other vascular beds it ranges from ϳ10 -15 s for small cremaster muscle arterioles (15-25 m) (613, 613, 615) to 60 -120 s for larger mesenteric and cremaster arterioles (ϳ100 m) (303, 740, 1671) and up to 5 min for mesenteric arteries and skeletal muscle arterioles (Ͼ180 m) (243, 667,1002). This diversity is probably related to a specific set of VSMC mechanosensation-contraction signaling systems.…”