Evidence for pH as an important parameter to control when studying the contractility of isolated rabbit cerebral arteries

Effects of long-term acidification of extracellular pH on ATP-induced calcium mobilization in rabbit lens epithelial cells

ATP-induced calcium (Ca2+) mobilization was investigated in rabbit lens epithelial cells that had been cultured in a medium with pH of 7.4 (group 1), 7.2 (group 2), or 7.0 (group 3) for 10 to 21 d. Intracellular free Ca2+ ([Ca2+]i and pH (pHi) were measured by using fluorescent dyes, fura-2 and BCECF, respectively. The long-term acidification decreased the pHi to 7.15 +/- 0.01, from 7.22 +/- 0.01, in group 2 and to 7.09 +/- 0.01 in group 3. The administration of 10 micromol/l ATP produced an initial peak followed by a sustained increase in [Ca2+]i in the lens cells of group 1. Both the initial peak and the sustained increase in [Ca2+]i were enhanced in groups 2 and 3. The initial peak was abolished by pretreatment with 1 micromol/l thapsigargin, an ER Ca2+ pump inhibitor, but was not affected by the removal of extracellular Ca2+. On the other hand, the sustained increase was suppressed either by the thapsigargin treatment or by the Ca2+ removal. Treatment with only thapsigargin caused a sustained increase in [Ca2+]i that was greater in group 3 than in group 1. These results suggest that (1) the ATP-induced initial peak in [Ca2+]i is due to Ca2+ release from the intracellular stores, (2) the sustained increase in [Ca2+]i is mediated through either Ca2+ influx from the extracellular space or Ca2+ release from the store triggered by the Ca2+ influx, and (3) long-term, moderate acidification enhances both the initial peak and the sustained increase in [Ca2+)]i in rabbit lens epithelial cells. One possible mechanism of the ATP-induced Ca2+ influx seems to be a capacitative Ca2+ entry pathway.

Extracellular pH, Ca(2+) influx, and response of vascular smooth muscle cells to 5-hydroxytryptamine

BACKGROUND AND PURPOSE

Cerebral vascular smooth muscle cells (VSMCs) contract on extracellular pH (pH(o)) increases and relax on pH(o) decreases. These changes in tone are believed to result from changes in [Ca(2+)](i), although the responsible mechanisms are not fully understood. VSMCs also contract in response to 5-hydroxytryptamine (5-HT), which increases [Ca(2+)](i) via both Ca(2+) release and influx. We hypothesized that examining effects of pH(o) decreases on 5-HT-induced [Ca(2+)](i) changes would allow us to identify mechanisms whereby pH(o) influences tone. Accordingly, we compared [Ca(2+)](i) increases in cerebral VSMCs, evoked by 5-HT, with increases evoked by increased pH(o) and examined 5-HT-dependent [Ca(2+)](i) increases at normal and decreased pH(o).

METHODS

We monitored [Ca(2+)](i,), using the Ca(2+)-sensitive dye fura 2, in cultured rat cerebral VSMCs obtained by enzymatic digestion of middle cerebral arteries and their branches (passages 1 to 3) grown on glass coverslips and superfused with physiological saline.

RESULTS

Increasing pH(o) from 7.3 to 7.8 increased [Ca(2+)](i), and these increases were prevented in Ca(2+)-free solutions. Decreasing pH(o) from 7.3 to 6.9 did not alter [Ca(2+)](i) unless [Ca(2+)](i) was first raised by treatment with 5-HT (10 micromol/L). 5-HT resulted in biphasic [Ca(2+)](i) increases characterized by transient peaks blocked by the Ca(2+)-ATPase inhibitor thapsigargin (10 nmol/L) and prolonged plateaus blocked by the Ca(2+) channel blocker Ni(2+) (1 mmol/L). Acidification did not alter the transient peaks but significantly reduced 5-HT-induced Ca(2+) influx.

CONCLUSIONS

We conclude that increasing pH(o) induces Ca(2+) influx in rat cerebral VSMCs and decreasing pH(o) inhibits 5-HT-stimulated Ca(2+) entry but not intracellular Ca(2+) release.

Mediation by 5-HT1D receptors of 5-hydroxytryptamine-induced contractions of rabbit middle and posterior cerebral arteries

1. 5-Hydroxytryptamine (5-HT) receptor-mediated contraction of endothelium denuded rabbit middle (MCA) and posterior (PCA) cerebral arteries was characterized by use of selective agonists and antagonists for different 5-HT receptor subtypes. 2. 5-HT and various 5-HT receptor agonists contracted the arteries with the following rank order of potency in MCA: 5-carboxamidotryptamine (5-CT) > 5-HT > 5-methoxytryptamine (5-MeOT) > sumatriptan > alpha-methyl-5-HT (alpha-Me-5-HT) >> 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and in PCA: 5-CT > 5-HT > sumatriptan > 5-MeOT > alpha-Me-5-HT >> 8-OH-DPAT. With few exceptions, the maximal contractile responses of these agonists were similar to that induced by 5-HT. 3. The selective antagonists of 5-HT2A/2C (ketanserin), 5-HT4 (SDZ 205-557) and 5-HT1A/1B (S-(-)-propranolol) sites were devoid of inhibitory effect on 5-HT-mediated contraction in both MCA and PCA, thus excluding activation of the corresponding receptors. 4. In both arteries, the contraction-response curve to 5-HT was unaffected by the 5-HT3 receptor antagonist, ICS 205-930 (0.01 and 0.1 microM) whilst a small (3 and 6 fold displacement) was seen with MDL 72222 (0.1 and 1 microM). 5. The mixed 5-HT1-like/5-HT2A receptor antagonist, methiothepin (0.001-0.1 microM), was a potent antagonist of 5-HT-induced contractions in both arteries, giving pA2 values of 9.4 +/- 0.7 and 9.6 +/- 0.8 in MCA and PCA, respectively. 6. Rauwolscine (O.1-10 MicroM) and yohimbine (0.3, 3 MicroM) inhibited contractions to 5-HT in a competitive manner, pA2 values of 7.1 +/- 0.6 and 6.7 +/-0.6 were determined for rauwolscine in MCA and PCA,respectively. An apparent pA2 value of 6.9 +/-0.2 was calculated for yohimbine (3 MicroM) in both MCA and PCA.7. In conclusion, these results suggest that the contractile response to 5-HT in rabbit isolated MCA and PCA is predominantly mediated by the 5-HTID receptor subtype, although a small contribution by 5-HT3 receptors cannot be excluded.