INCREASED RESTING CA2+ MAINTAINS THE MYOGENIC TONE AND ACTIVATES K+ CHANNELS IN ARTERIES FROM YOUNG SPONTANEOUSLY HYPERTENSIVE RATS

We examined whether the Ca2+ channel function in the resting state alters the resting tone in femoral and carotid arteries from spontaneously hypertensive rats (SHR) at early hypertensive stages (6 and 4 weeks of age), and data were compared with findings in age-matched normotensive Wistar-Kyoto rats (WKY). Strips of femoral and carotid arteries from 6-week-old SHR, but not from WKY, maintained a myogenic tone, that is, the resting tone decreased when 10(-7) M nifedipine was added. A similar myogenic tone was maintained in 4-week-old SHR. In strips of carotid arteries preloaded with fura-2, a fluorescent Ca2+ indicator, the decrease in cytoplasmic Ca2+ concentration following 0-Ca2+ solution or 5 x 10(-7) M nicardipine was significantly greater in SHR than in WKY. The basal Ca-45 influx in femoral and carotid arteries from 6-week-old SHR was significantly increased when compared with WKY, and this increase in SHR was abolished by 10(-7) M nifedipine. The addition of charybdotoxin (a blocker of large conductance Ca2+-activated K+ channels) or of Bay k 8644 (an agonist of L-type voltage-dependent Ca2+ channels; VDCs), caused a concentration-dependent contraction, which was significantly greater in 6- and 4-week-old SHR than in WKY. These results suggest that the Ca2+ influx via L-type VDCs was increased in the resting state of femoral and carotid arteries from SHR at the early hypertensive stages, and therefore the myogenic tone was maintained and charybdotoxin-sensitive K+ channels were highly activated.