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R Bakker

Publications and source records attributed to R Bakker.

25 records · Page 2Linked to original sources
PubMed1989

Serotonergic neurons in the intestine of two teleosts, Carassius auratus and Oreochromis mossambicus, and the effect of serotonin on transepithelial ion-selectivity and muscle tension.

The application of an antiserum directed against rat serotonin demonstrated serotonin-immunoreactive cell bodies and varicose nerve fibres in the myenteric plexus of both goldfish and tilapia. In the circular muscle layer immunoreactive varicose nerve fibres and a few cell bodies could be detected. A fine network of varicose fibres was observed underlying the epithelial cells. Serotonin immunoreactivity was not observed along incoming mesenterial blood vessels, suggesting that the serotonergic neurons may be intrinsic to the intestine. Immunoreactive endocrine cells were shown in the intestinal epithelium of tilapia but not in goldfish. Serotonin caused a weak contraction of the intestinal wall of the goldfish which could be blocked by tetrodotoxin and by atropine, suggesting that serotonin has an indirect action on muscular contraction. In contrast, serotonin induced a relaxation of the intestinal wall of tilapia which could not be blocked by propranolol or by tetrodotoxin. This indicates that in this species serotonin may act directly on the muscle fibres. The ion-selectivity of the intestinal epithelium of both species was modulated by serotonin. Tetrodotoxin did not inhibit this effect, suggesting that serotonin acts directly on the epithelial cells. The presence of serotonergic fibres in the muscle layer and directly underneath the epithelium, along with the effect of serotonin on muscular tension and on the ion-selectivity of the epithelium, suggests that serotonin may play a role in the regulation of motility and the epithelial function of goldfish and tilapia intestine.

Animals↗
PubMed1986

Transmission of X-ray-induced reciprocal translocations in normal male mice and in male mice with a reduced sperm count due to translocation homozygosity.

Normal (+/+) and translocation T(1; 11.13S)70H homozygous (T/T) male mice received 2 X 2.5 Gy X-rays with a 24-h interval. After 120 days, the frequency of late diplotene-metaphase I spermatocytes with translocation multivalents was 14.1% for +/+ and 13.7% for T/T males, respectively, in one group of animals of each type. The difference is not significant. A second group was allowed to sire progeny for 60 days with 2 normal females per week. Reciprocal translocations detectable at diakinesis/metaphase I were observed in 2.5% of the 395 male progeny from the irradiated +/+ fathers, and in 2.9% of the 489 male progeny from the irradiated T/T fathers. This leads to a pooled estimated transmission of 0.81 +/- 0.19. Translocations induced in the long 11.13 metacentric chromosome were not transmitted with a different frequency. The rate of heritable induced translocations in this study was 5.4 X 10(-5)/rad/gamete. On the basis of the data of Generoso et al. (1984) for the frequency of the heritable spontaneous translocations in male mice, it is concluded that, because of their low doubling dose (3.3-4.6 rad), the spontaneous translocations are probably of postmeiotic origin.

Animals↗
PubMed1984

cAMP-mediated effects of ouabain and theophylline on paracellular ion selectivity.

We studied the effects of theophylline and ouabain on isolated intestinal mucosa of the goldfish. From continuous recordings of the intracellular potential difference, we concluded that theophylline has no considerable effect on the Cl- conductance of the apical membrane. Theophylline increased transepithelial conductance and bidirectional Cl- fluxes, while Na+ fluxes did not change appreciably. Both streaming potentials and NaCl dilution potentials were immediately reduced by theophylline. Moreover, we found a striking similarity between the effects of theophylline and ouabain. Ouabain initially increased transepithelial conductance, increased bidirectional Cl- fluxes, and instantly abolished NaCl dilution potentials. Determinations of cellular cAMP levels revealed that both theophylline and ouabain increase cAMP. The phosphodiesterase inhibitor theophylline amplified the ouabain-induced rise in cAMP, suggesting that ouabain stimulates adenylate cyclase. Since elevated extracellular K+ did not affect cAMP levels, it can be ruled out that depolarization of the cell membrane due to ouabain increases cAMP. We conclude that in this epithelium both theophylline and ouabain increase the Cl- permeability of the paracellular pathway by a cAMP-dependent mechanism.

Animals↗
PubMed1981

Analysis of the ouabain-induced increase in transepithelial electrical resistance in the goldfish intestinal mucosa.

1. The ouabain-induced increase in transmural resistance of goldfish intestinal mucosa stripped free from underlying muscular layers is analysed by comparing the resistance increase in normal and in low chloride saline, the resistance increased induced by anaerobic conditions and the resistance increase provoked by hypotonicity. 2. It is concluded that the collapse of the lateral intercellular space is the prime reason for the resistance increase and that the lateral intercellular space is maintained dilated by a ouabain-sensitive solute transport mechanism. 3. This mechanism can be either a rheogenic or a neutral Na/K-pump. In the latter case additional conditions have to be specified concerning values for ion concentrations in the lateral intercellular space and in the unstirred layer adjacent to the luminal membrane. 4. There are no indications for a chloride dependent mechanism involved in the maintenance of the width of the lateral intercellular spaces in the goldfish intestinal mucosa.

Animals↗