Physiological variations in ovine cerebrovascular calcium sensitivity

Abstract

Cerebrovascular reactivity to biogenic amines varies in relation to both arterial diameter and age. The present study examines the hypothesis that these patterns of reactivity are secondary to corresponding variations in the Ca2+ sensitivity of the contractile proteins. To test this hypothesis, we permeabilized segments of common carotid (Com), basilar, main branch middle cerebral, and second-branch middle cerebral (MCA-B) arteries from nonpregnant adult and near-term fetal sheep using beta-escin. Permeabilization methods were carefully validated and adjusted for each artery type. Baseline myofilament Ca2+ sensitivity in both adults and fetuses increased significantly from the Com to the MCA-B and was generally higher in fetuses than in adults. Serotonin dose dependently increased Ca2+ sensitivity via a G protein-dependent mechanism in all arteries. The magnitudes of this effect did not vary among artery types but were significantly greater in fetal than in adult arteries. This effect of serotonin was mimicked by guanosine 5'-O-(3-thiotriphosphate), a nonhydrolyzable analog of guanosine 5'-triphosphate, and its effects were also much greater in fetal than in adult arteries. We conclude that patterns of cerebrovascular reactivity to biogenic amines were determined, at least in part, by underlying variations in baseline myofilament Ca2+ sensitivity and/or its alteration by G protein-dependent mechanisms.

Developmental differences in Ca2+-activated K+channel activity in ovine basilar artery

“…patch clamp; perforated whole cell; inside-out patch; calcium set point; calcium-activated potassium channel FROM A FUNCTIONAL STANDPOINT, several developmental differences have been reported for vascular smooth muscle (VSM). These include the resting membrane potential (12), intracellular calcium regulation (6,24), the sensitivity of contraction to calcium concentration (1), and many others (31).…”

Modulation of BK channel calcium affinity by differential phosphorylation in developing ovine basilar artery myocytes

“…For fetal myocytes, the lower Ca 0 values imply that, at a given [Ca 2ϩ ] i and voltage, these channels are more activated than those of the adult. We propose that this lower Ca 0 value in the fetus is an adaptation to a lower resting [Ca 2ϩ ] i (1,12,18) and to a higher resting membrane potential (10, and present findings) to compensate for greater dependence on extracellular Ca 2ϩ (5,24). In ovine cerebral arteries, maturation is associated with decreased Ca 2ϩ release from ryanodine receptor channels (24,27).…”

“…These include differences in K ϩ channel expression (35), K ϩ channel activity (18,23), resting membrane potential (10), intracellular Ca 2ϩ regulation (5,24), sensitivity of contraction to [Ca 2ϩ ] i (1,12,25), and others (26,32). In the ovine fetal middle cerebral artery, contraction depends almost completely on the influx of extracellular Ca 2ϩ , whereas contraction in adult myocytes relies more on Ca 2ϩ release from intracellular Ca 2ϩ stores, including the inositol 1,4,5-trisphosphate-releasable and ryanodine-sensitive stores (1,24,27).…”

Effects of Maturation on Mechanisms of cGMP-Induced Cerebral Vasodilatation

“…Arterial segments were equilibrated in a calcium-EGTA buffer that contained (in mM ): 5 EGTA, 5 ATP, 110 potassium acetate, 6 magnesium acetate, 1 dithiothreitol and 20 HEPES, at pH 6.8 (titrated with KOH) at 25°C as previously described [3,29].…”

“…These buffers were prepared by mixing various ratios of calibrated zero and 10 ÌM calcium buffers, as previously described in detail [3]. Then, the arteries were incubated for 40 min with the EC 90 concentration of 8-pCPT-cGMP, after which the calcium-concentration-response determinations were repeated.…”