Role of Ca 2 1 channels in NE-induced increase in [Ca 2 1 ] i and tension in fetal and adult cerebral arteries

Role of Ca 2 1 channels in NE-induced increase in [Ca 2 1 ] i and tension in fetal and adult cerebral arteries. Am. J. Physiol. 277 ( Regulatory Integrative Comp. Physiol. 46): R286–R294, 1999. —In vascular smooth muscle, elevation of agonist-induced intracellular Ca 2 1 concentration ([Ca 2 1 ] i ) occurs via both Ca 2 1 release from intracellular stores and Ca 2 1 inﬂux across the plasma membrane. In the cerebral vasculature of the fetus and adult the relative roles of these mechanisms have not been deﬁned. To test the hypothesis that plasma membrane L-type and receptor-operated Ca 2 1 channels play a key role in NE-induced vasoconstriction via alterations in plasma membrane Ca 2 1 ﬂux and that this may change with developmental age, we performed the following study. In main branch middle cerebral arteries (MCA) from near-term fetal ( , 140 days) and nonpregnant adult sheep, we quantiﬁed NE-induced responses of vascular tension and [Ca 2 1 ] i (by use of fura 2) under standard conditions in response to several Ca 2 1 channel blockers and in response to zero extracellular Ca 2 1 . In fetal and adult MCA, maximal NE-induced tensions (g) were 0.91 6 0.12 ( n 5 10) and 1.61 6 0.13 ( n 5 12), respectively. The pD 2 values for NE-induced tension were both 6.0 6 0.1, whereas the fetal and adult maximum responses (%K max ) were 107 6 16 and 119 6 7, respectively. The fetal and adult pD 2 values for NE-induced increase of [Ca 2 1 ] i were 6.2 6 0.1 and 6.4 6 0.1, respectively, whereas maximum [Ca 2 1 ] i responses were 81 6 9 and 103 6 15% of K max , respectively. After 10 2 5 M NE-induced contraction, nifedipine resulted in dose-dependent decrease in vessel tone and [Ca 2 1 ] i with pIC 50 values for fetal and adult tensions of 7.3 6 0.1 and 6.6 6 0.1, respectively ( P , 0.01; n 5 4 each), whereas pIC 50 for [Ca 2 1 ] i responses were 7.2 6 0.1 and 6.9 6 0.1, respectively. The pIC 50 values for tension for diltiazem and verapamil were somewhat lower but showed a similar relationship. The receptor-operated Ca 2 1 channel blocker 2-nitro-4 carboxyphenyl- N,N -diphenyl carbamate showed little effect on NE-induced vessel contractility or [Ca 2 1 ] i . In the absence of extracellular Ca 2 1 for 2 min, 10 2 5 M NE resulted in markedly attenuated responses of adult MCA tension and [Ca 2 1 ] i to 39 6 7 and 73 6 8% of control values ( n 5 4). For fetal MCA, exposure to extracellular Ca 2 1 concentration resulted in essentially no contractile or [Ca 2 1 ] i response ( n 5 4). Similar blunting of NE-induced tension and [Ca 2 1 ] i was seen in response to 10 2 3 M lanthanum ion. These ﬁndings provide evidence to suggest that especially in fetal, but also in adult, ovine MCA, Ca 2 1 ﬂux via L-type calcium channels plays a key role in NE-induced contraction. In contrast, Ca 2 1 ﬂux via receptor-operated Ca 2 1 channels is of less importance. This developmental difference in the role of cerebrovascular plasma membrane Ca 2 1 channels may an important association with increased Ca 2 1 sensitivity of the fetal vessels. tension for the studies, we performed K 1 induced contractility experiments with resting tension rang-ing from 0.2 to 0.5 g ( n 5 4 each). In both fetal and adult arteries, optimal results for both tension and ﬂuorescence ratio were obtained at 0.3 g, the value chosen for the present studies. The vessel inside diameter measurements, in combi-nation with measurements of vessel wall thickness, length, and potassium-induced force, enabled calculation of force per unit cross-sectional area, as previously described (30, 31). ANOVA (vessel, age) coupled with Duncan’s multiple range test was used. Where appropriate, we used ANOVA with repeated measures. The correlation between tension and ﬂuorescence ratio was determined by linear regression (8). A P value of , 0.05 was considered signiﬁcant.