Mechanisms of a-adrenergic enhancement of the myogenic response

Abstract

Mechanisms of interaction between α-adrenergic and myogenic mechanisms were studied in the intact microcirculation of the rat cremaster muscle. Anesthetized rats were enclosed in an airtight box which could be pressurized to increase intravascular pressure in the cremaster. The vessel diameter, intravascular pressure and red cell velocity were measured in the first order arteriole (1A) (114 [plus or minus] 13 μm, mean [plus or minus] SD) during box pressure increases of 10, 20 and 30 mmHg. The results indicate that activation of α-adrenoceptors by norepinephrine (NE, 2x10^-7 M - 2x10^-6 M) results in significant enhancement of myogenic constriction. This NE-enhanced response was inhibited by the potential-operated Ca[^2+] channel (POC) antagonist nifedipine (10^-6 M or 10^-5 M), and potentiated by the POC agonist Bay K 8644 (5x10^-7 M). Bay K 8644 alone also potentiated the myogenic response. The involvement of the second messenger pathway leading to protein kinase C (PKC) activation was also investigated. G-protein activation with AIF3 (2x10^-3 M NaF & 2x10^-5 M AICI3) caused 15% constriction of the 1A and facilitated myogenic constriction. The AIF3 vasoconstriction was inhibited by phospholipase C (PLC) inhibition with neomycin (10^-3 M) or 2-nitro-4-carboxyphenyl N,N-diphenylcarbamate (NCDC) (10^-4 M) and with the PKC inhibitor calphostin C (5x10^-7 M). These data suggest that G-protein activation potentiates myogenic responsiveness and involves activation of PKC. PLC inhibition with NCDC also significantly attenuated the NE-enhanced myogenic response, implicating the involvement of PLC. Inhibitors of PKC, staurosporine (10^-7 M) and calphostin C (10^-6 M), also significantly attenuated the NE-enhanced myogenic response. PKC activation with indolactam (10^-6 M) was found to increase vascular tone in the 1A (109 [plus or minus] 6 to 89 [plus or minus] 7 μm) and to enhance myogenic responsiveness. These results suggest that NE acts in part through activation of PKC. Collectively, the results indicate that both POC and the intracellular signaling pathway involving PLC and PKC are involved in the α-adrenergic enhancement of myogenic activity. These two mechanisms appear to represent overlap in the signaling pathways for α-adrenergic and myogenic activation of vascular smooth muscle.