Release of prostaglandins during the contraction of rat aorta induced by ouabain and K+-free solution

Changes in the isoforms of the sodium pump in the placenta and myometrium of women in labor

OBJECTIVE

We determined whether changes in sodium pump isoform abundance accompanied active human labor.

STUDY DESIGN

Specimens of placenta, amniochorion, and myometrium were collected from women in active spontaneous labor and from those not in labor. The abundance of the three sodium pump alpha-isoforms was determined by Western blot analysis.

RESULTS

Levels of the alpha1 and alpha2 isoforms were comparable in the three tissues for women in labor and not in labor. However, alpha3 isoform abundance in placenta and myometrium (but not amniochorion) was significantly decreased in women in active labor compared with women not in labor (sodium pump alpha3 in placenta: no labor 91.2 +/- 27.6 vs labor 46.9 +/- 3.6 density units, P =.002. Sodium pump alpha3 in myometrium: no labor 52.3 +/- 7.7 vs labor 19.8 +/- 1.6 density units, P =.0002).

CONCLUSION

Because reductions in sodium pump number can result in hormone release from secretory tissues and in contraction of muscle, this suggests that the sodium pump may play a significant role in the initiation or maintenance of human labor.

Contractile and relaxant effects of phorbol ester in the intestinal smooth muscle of guinea-pig taenia caeci

1. Effects of phorbol esters on the cytosolic Ca2+ level ([Ca2+]i) and muscle tension in the intestinal smooth muscle of guinea-pig taenia caeci were examined. 2. 12-Deoxyphorbol 13-isobutyrate (DPB, 1 microM) did not change the [Ca2+]i and tension in resting muscle. 3. In high K(+)-stimulated muscle, 1 microM DPB transiently augmented the contraction and decreased [Ca2+]i. 12-Deoxyphorbol 13-isobutyrate 20-acetate (1 microM) and phorbol 12, 13-dibutyrate (1 microM) showed similar effects to DPB whereas phorbol 12-myristate 13-acetate (1 microM) and phorbol 12, 13-didecanoate (1 microM) were ineffective. 4. DPB (1 microM) inhibited both [Ca2+]i and tension stimulated by 300 nM carbachol or 3 microM histamine. In the presence of a higher concentration of carbachol (1 microM), DPB decreased [Ca2+]i and transiently increased muscle tension. 5. In the muscle strips permeabilized with bacterial alpha-toxin, 1 microM DPB shifted the Ca(2+)-tension curve to the left. An inhibitor of protein kinase C, H-7 (30 microM), inhibited the effect of DPB. 6. DPB did not change the high K(+)-induced contraction in the muscle strips pretreated with 3 microM phorbol 12-myristate 13-acetate for 24 h. 7. These results suggest that activation of protein kinase C has dual effects; it augments contraction by increasing the Ca2+ sensitivity of the contractile elements and it inhibits contraction by decreasing [Ca2+]i.

45Ca2+ movements induced by Ca2+ chloride in isolated rat aorta under K(+)-free conditions

Increasing the extracellular Ca2+ concentration induced a dihydropyridine-insensitive contraction in the isolated rat aorta bathed in K(+)-free solution. To obtain further insight into the mechanism of this contraction 45Ca2+ uptake measurements were carried out with isolated rat aorta. Increasing the extracellular Ca2+ concentration from 0.63 to 5.0 mM enhanced the 45Ca2+ uptake from 186 +/- 3 to 359 +/- 12 dpm/mg ww (n = 8). Under K(+)-free conditions increasing the extracellular Ca2+ concentration from 0.63 to 5.0 mM enhanced the 45Ca2+ uptake from 184 +/- 4 to 541 +/- 6 dpm/mg ww (n = 12) and elicited an increase in tension of 10.0 +/- 0.4 mN (n = 9). The Ca2(+)-induced 45Ca2+ uptake was not affected by the Ca2% channel antagonists, nifedipine (L-type), flunarizine (L- and T-type) and omega-conotoxin (N-type). Amiloride, a blocker of the Na+/Ca2+ exchange, indomethacin, a blocker of prostaglandin synthesis, the local anaesthetic, lidocaine, and the Ca2+ overload blocker. R 56865, had no effect on Ca2(+)-induced 45Ca2+ uptake. Additionally, R 56865 did not affect the Ca2(+)-induced contractile response. The inorganic Ca2+ entry blocker, lanthanum, reduced both the non-stimulated and stimulated 45Ca2+ uptake and inhibited the Ca2(+)-induced contractile response. We would like to suggest that the 45Ca2+ uptake induced by Ca2+ under K(+)-free conditions enters the cell via the leakage Ca2+ channel, because lanthanum was an effective antagonist and a significant contribution of any other Ca2+ pathway could not be demonstrated.