Changes in Myogenic Tone in Spontaneously Hypertensive Rat: Role of RhoA and Protein Kinase C

BACKGROUND AND OBJECTIVES: The myogenic response was originally described as a contraction of a blood vessel that occurred following an increase in intravascular distending pressure. Conversely, a reduction in intravascular pressure produces myogenic vascular relaxation. Recent attention has focused on the potential role of this myogenic mechanism in the control of tone in the resistance vasculature, and in particular on how this mechanism may contribute to the increased vascular resistance seen in hypertension. Therefore, in the present study, we investigated the role of myogenic tone in the generation and/or maintenance of hypertension. MATERICAL AND METHODS: Myogenic tone was developed by stretching of the basilar arteries of WKY (istar Kyoto rat) and SHR (spontaneously hypertensive rats). Contractile responses, PKC (protein kinase C) immunoblots and translocation of PKC and RhoA were measured. In the presence of extracellular Ca2+ the stretching of the resting vessel evoked a myogenic contraction in the basilar arteries of SHR and WKY. Myogenic tone was significantly greater in SHR than in WKY. However, in the absence of extracellular Ca2+, stretching evoked a myogenic contraction in SHR, but not in WKY. The stretch-induced myogenic tone was inhibited by nifedipine. The effect of nifedipine was similar in both SHR and WKY rats. H-7, calphostin C and Y-27632, also inhibited stretch-induced myogenic tone in both SHR and WKY. The inhibitory effects of these drugs were greater in SHR than in WKY. Immunoblotting showed rho A and PKC alpha were translocated from the cytosol to the cell membrane with stretching in both SHR and WKY. PKC beta, however, was translocated to the cell membrane with stretching in SHR, but not in WKY. CONCLUSION: These results suggest that stretch-induced myogenic tone is significantly greater in SHR than in WKY. Furthermore, the increase in amount and/or activity of PKC beta and ROK (rhoA-associated kinase) may be a key mechanism accounting for the enhanced myogenic tone in SHR.

Ca2+ Sensitization Mechanism in Stretch-induced Myogenic Tone

It has been suggested that Ca2+ sensitization mechanisms might contribute to myogenic tone, however, specific mechanisms have not yet been fully identified. Therefore, we investigated the role of protein kinase C (PKC)- or RhoA-induced Ca2+ sensitization in myogenic tone of the rabbit basilar vessel. Myogenic tone was developed by stretch of rabbit basilar artery. Fura-2 Ca2+ signals, contractile responses, PKC immunoblots, translocation of PKC and RhoA, and phosphorylation of myosin light chains were measured. Stretch of the resting vessel evoked a myogenic contraction and an increase in the intracellular Ca2+ concentration ([Ca2+]i) only in the presence of extracellular Ca2+. Stretch evoked greater contraction than high K+ at a given [Ca2+]i. The stretch-induced increase in [Ca2+]i and contractile force were inhibited by treatment of the tissue with nifedipine, a blocker of voltage-dependent Ca2+ channel, but not with gadolinium, a blocker of stretch-activated cation channels. The PKC inhibitors, H-7 and calphostin C, and a RhoA-activated protein kinase (ROK) inhibitor, Y-27632, inhibited the stretch-induced myogenic tone without changing [Ca2+]i. Immunoblotting using isoform-specific antibodies showed the presence of PKCalpha and PKCepsilon in the rabbit basilar artery. PKCalpha, but not PKCepsilon, and RhoA were translocated from the cytosol to the cell membrane by stretch. Phosphorylation of the myosin light chains was increased by stretch and the increased phosphorylation was blocked by treatment of the tissue with H-7 and Y-27632, respectively. Our results are consistent with important roles for PKC and RhoA in the generation of myogenic tone. Furthermore, enhanced phosphorylation of the myosin light chains by activation of PKCalpha and/or RhoA may be key mechanisms for the Ca2+ sensitization associated with myogenic tone in basilar vessels.