ALPHA(1A)-ADRENERGIC RECEPTORS MEDIATE VASOCONSTRICTION OF THE ISOLATED SPIRAL MODIOLAR ARTERY IN-VITRO

Several lines of evidence suggest that cochlear blood flow is under th e control of the sympathetic nervous system and that this control is m ediated via alpha-adrenergic receptors. The goal of the present study was to determine whether alpha-adrenergic receptors mediate vasoconstr iction of the spiral modiolar artery and, if so, to determine which su btype dominates this response. Vascular diameter was measured with vid eo microscopy in the isolated superfused spiral modiolar artery in vit ro. The diameter of the spiral modiolar artery under control condition s was 61+/-2 mu m (n=60). Spontaneous vasomotion was observed in most specimens. Addition of norepinephrine to the superfusate caused a phas ic vasoconstriction and an increase in the amplitude of vasomotion. Th ese effects were limited to the vicinity of arteriolar branch points o f the spiral modiolar artery. Norepinephrine-induced vasoconstriction occurred with EC50 of (1.9+/-0.4)X10(-5) M (n=44) and the vascular dia meter was maximally reduced by a factor of 0.87+/-0.01 (n=29). Neither the phasic nature nor the EC50 of the norepinephrine-induced vasocons trictions was altered in the presence of the beta(2)-adrenergic recept or antagonist 10(-5) M ICI118551 or the nitric oxide synthase inhibito r 10(-4) M NOARG. In contrast, the alpha(2)-adrenergic receptor antago nist 10(-7) M yohimbine and the alpha(2)-adrenergic receptor antagonis t 10(-9) and 10(-8) M prazosin caused a significant shift in the dose- response curve. The affinity constants (K-DB) for yohimbine and prazos in were (5+/-2)X10(-8) M (n=4) and (2.0+/-0.7)X10(-10) M (n=18), respe ctively. The alpha(1A)-adrenergic receptor antagonist 10(-8) M 5-methy l urapidil and the alpha(1D)-adrenergic receptors antagonist 5X10(-6) M BMY7378 caused a significant shift in the dose-response curve. The K -DB values for 5-methyl urapidil and for BMY7378 were (2.7+/-0.7)X10(- 10) M (n=8) and (4.4+/-2.7)X10(-7) M (n=8), respectively. Further, tot al RNA was isolated from microdissected spiral modiolar arteries and t he presence of transcripts for alpha(1)-adrenergic receptor subtypes w as determined by reverse transcription polymerase chain reaction (RT-P CR). Primers specific for gerbil alpha(1)-adrenergic receptor subtypes were developed using RNA from rat and gerbil brain. Analysis of RNA e xtracted from the spiral modiolar artery revealed RT-PCR products of t he appropriate size for the alpha(1A)-adrenergic receptor, however, no evidence for the alpha(1B)- and alpha(1D)-adrenergic receptor was fou nd. Further, analysis of RNA extracted from blood, which was a contami nant of the microdissected spiral modiolar arteries, revealed no RT-PC R products. Sequence analysis of the RT-PCR product of the alpha(1A)-a drenergic receptor from the spiral modiolar artery confirmed its ident ity. Identity between the 175 nt gerbil sequence fragment and the know n rat, mouse and human alpha(1A)-adrenergic receptor sequences was 90. 9, 92.0 and 85.2%, respectively. These observations demonstrate that t he spiral modiolar artery contains alpha(1A)-adrenergic receptors whic h mediate vasoconstriction at branch points. (C) 1998 Elsevier Science B.V.