Serotonin increases the cAMP concentration and the phosphoenolpyruvate carboxykinase mRNA in rat kidney, small intestine, and liver.

Within 60 min of the administration of serotonin to fasted-refed rats, there was a 5-, 16-, and 20-fold stimulation of the mRNA coding for the cytosolic form of P-enolpyruvate carboxykinase in the kidney, small intestine and liver, respectively. This stimulation was 5-, 1.3-, and 2-fold higher than noted in the same tissue after 24 h of starvation. Dose- and time-response curves to serotonin in the three tissues were similar. The level of PEPCK mRNA in the liver was significantly elevated within 30 min of serotonin administration, whereas 60 min was required in the small intestine and the kidney. The direct effect of serotonin on PEPCK mRNA was also assessed in hepatocytes maintained in primary culture. Serotonin (10(-8) M to 10(-4) M) caused a dose-dependent increase in the level of PEPCK mRNA and a transient increase in cAMP concentration. Within the first min of serotonin (10(-6) M) addition to cells, cAMP concentration increased 4-fold and returned after 10 min to basal level. Therefore, these results provide functional evidence of serotonin action in the rat peripheric tissues and suggest that cAMP is involved in its intracellular signalling.

Prolactin increases cytosolic free calcium concentration in hepatocytes of lactating rats.

PRL at a physiological concentration (10(-8) M) produced a very rapid and transient increase in 45Ca efflux in freshly isolated hepatocytes, which reached the highest value within 5 min and returned to baseline level after 20 min. PRL-induced 45Ca2+ efflux resulted in a loss of 15% of total cell calcium, which was similar to that found in vasopressin-treated cells. However, in contrast with the PRL effect, 45Ca2+ efflux induced by vasopressin was sustained. We demonstrate by using two different approaches, glycogen phosphorylase-a activation and direct cytosolic calcium concentration [( Ca2+]i) measurements, that PRL elicits a [Ca2+]i increase. The treatment of hepatic cells with PRL caused a 4-fold stimulation in glycogen phosphorylase-alpha activity after 2 min of PRL addition. Direct [Ca2+]i determination in fluo-3-loaded hepatocytes showed a 11% increase after 5 min of PRL addition. Similar data were observed in hepatocytes stimulated either with vasopressin (10(-7) M) or calcium ionophore A23187 (200 nM). The increase in [Ca2+]i promoted by PRL was independent of extracellular calcium or voltage-operated calcium channels. The data demonstrate that calcium is involved in the intracellular signaling of PRL in liver cells and that PRL initiates its action by a Ca2+ mobilization from the intracellular stores.

Regulation of expression of the messenger ribonucleic acid encoding the cytosolic form of phosphoenolpyruvate carboxykinase in liver and small intestine of lactating rats.

The content of P-enolpyruvate carboxykinase enzyme mRNA (mRNA(PEPCK)) in the liver and small intestine changes in a coordinate way at various stages of pregnancy, lactation, and weaning periods of rats. On the ninth day of lactation, the accumulation of mRNA(PEPCK) in the liver and small intestine was 82% and 152%, respectively, compared with that in starved control rats. After 18 h of induced weaning, both tissues presented residual levels of mRNA(PEPCK). However, after 60 h of induced weaning, the accumulation of mRNA(PEPCK) increased to 60% in the liver and 76% in the small intestine compared with that in starved control rats. Adrenalectomy and bromocriptine-induced hypoprolactinemia decreased the content of this mRNA in the liver of lactating rats, while diabetes increased it. PEPCK gene transcription, determined in isolated nuclei, was 2.4-fold higher in the liver of lactating rats than in hypoprolactinemic animals, but 3.7-fold lower than that in starved animals. These results suggest that the mRNA(PEPCK) accumulation observed in the liver of lactating rats must be due to an increase in both transcription rate and messenger stabilization. PRL added to primary culture of hepatocytes obtained from lactating rats caused a dose-dependent increase in the level of mRNA(PEPCK) and mRNAc-myc. Therefore, the high PEPCK activity reported during lactation is due to increased amounts of mRNA(PEPCK), which, in turn, is due to a coordinated response at transcriptional and posttranscriptional levels, where PRL seems to play an important role.

Regulation of lipid metabolism by dipyridamole and adenosine antagonists in rat adipocytes.

1. Acute effects of dipyridamole, an inhibitor of adenosine transport, direct activators of adenylate cyclase and thirteen adenosine antagonist analogs on fatty acid synthesis have been examined in terms of the control of [1-14C]acetate incorporation into labeled fatty acids in the presence of glucose. 2. This monosaccharide acts as a stimulator of lipogenesis by generating NADPH for the lipid synthesis. 3. The relationship between lipogenesis and lipolysis was compared with a variety of adenylate cyclase stimulators. 4. The data obtained reveals that dipyridamole potentiated the inhibitory or stimulatory effects of isoproterenol and forskolin on lipogenesis and on lipolysis, respectively. 5. In these cases the data show that it exists an inverse relationship between lipogenesis and lipolysis. 6. Dipyridamole and methylxanthine analogs only moderately affect the rate of lipolysis whereas its effects are more potent on lipogenesis and lend further support to the hypothesis that dipyridamole antagonize adenosine actions as well as methyl xanthines. 7. These results suggest that dipyridamole and adenosine antagonists alter lipogenesis independently of the lipolytic process and that it exists an inverse relationship between lipogenesis and lipolysis under some conditions whereas there are not under others.