title: Differential activation of alpha 1- and alpha 2-adrenoceptors on microvascular smooth muscle during sympathetic nerve stimulation.
abstract: The relative contribution of postjunctional alpha 1- and alpha 2-adrenoceptors to constriction of microvessels was examined during sympathetic nerve stimulation and sympathetic escape (difference between peak and steady-state constriction). Large arterioles (120 +/- 4 microns control diameter) and venules (174 +/- 6 microns) and small arterioles (13 +/- 4 microns) were examined in rat cremaster skeletal muscle during stimulation of the cremaster efferent innervation (decentralized lumbar sympathetic chain, 0.5-16 Hz, 2-minute train). The muscle was suspended in a tissue bath, and diameter was measured with intravital microscopy. Frequency-response curves were obtained after vehicle (prazosin or rauwolscine) was added to the bath. In large arterioles, prazosin (10(-7) M) significantly attenuated constriction by 60-80%; a fivefold higher concentration had no additional effect. In contrast, rauwolscine (1 to 5 x 10(-7) M) had no effect. Venules evidenced minimal response to nerve stimulation. In small arterioles, rauwolscine (5 x 10(-7) M) significantly attenuated constriction by 50-60%, while prazosin (10(-7) M) had no effect. These data suggest that for large arterioles, which are known to possess both receptors, alpha 1-adrenoceptors are preferentially stimulated by nerve-released norepinephrine. In contrast, sympathetic constriction of small arterioles is mediated by alpha 2-adrenoceptors. Compared with large arterioles, small arterioles exhibited greater peak and steady-state constriction at all frequencies, with maximal responses achieved over the 0.5-4 Hz range. Large arterioles exhibit graded constriction over the entire frequency range. Sympathetic escape exhibited a small, negatively correlated frequency dependence for large arterioles, tended to be greater for small arterioles, and was more evident in large arterioles during alpha 2-adrenoceptor constriction at low-frequency stimulation. This distinct neural control of large resistance vessels by alpha 1-adrenoceptors and small terminal arterioles by alpha 2-adrenoceptors may allow neurogenic regulation of these vessel segments to be differentially susceptible to modulation by other extrinsic and intrinsic vasoactive controls that preferentially interact with alpha 1- and alpha 2-adrenergic contractile mechanisms.
authors.affiliation: Department of Physiology, University of North Carolina, Chapel Hill 27599-7545.
authors.forename: M
authors.initials: M
authors.lastname: Ohyanagi
authors.forename: J E
authors.initials: JE
authors.lastname: Faber
authors.forename: K
authors.initials: K
authors.lastname: Nishigaki
mesh_terms: D000818:Animals; D001160:Arterioles / Q000378:metabolism* / Q000502:physiology; D004558:Electric Stimulation; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D013564:Sympathetic Nervous System / Q000502:physiology*; D014699:Venules / Q000378:metabolism* / Q000502:physiology; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D011224:Prazosin