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Biomedical subjects

M D Levitt

Publications and source records attributed to M D Levitt.

At least 217 records · Page 12Linked to original sources
PubMed1973

Use of inert gases to study the interaction of blood flow and diffusion during passive absorption from the gastrointestinal tract of the rat.

Measurement of the relative absorption rates of inert gases (H(2), He, CH(4), SF(6), and (133)Xe) was used to investigate the interaction between diffusion and blood flow during passive absorption from the stomach, small bowel, and colon of the rat. If uptake is blood flow limited, the gases should be absorbed in proportion to their solubilities in blood, but if diffusion limited, uptake should be proportional to the diffusion rate of the gases in mucosal tissues. The observed absorption data were fitted to a series of models of interaction between perfusion and diffusion. A simple model accurately predicted the absorption rates of the gases from all segments of bowel. In this model, gas is absorbed into two distinct blood flows: one which flows in proximity to the lumen and completely equilibrates with the lumen, and a second which is sufficiently rapid and distant from the lumen that its gas uptake is entirely diffusion limited. The fraction of the total absorption attributable to the equilibrating flow can be readily calculated and equalled 93%, 77%, and 33% for the small bowel, colon, and stomach, respectively. Thus the rate of passive absorption of gases from the small bowel is limited almost entirely by the blood flow to the mucosa, and absorption from the stomach is largely limited by the diffusion rate of the gases. The flow which equilibrates with the lumen can be quantitated, and this flow may provide a useful measure of "effective" mucosal blood flow.

Animals↗
PubMed1972

Use of pulmonary hydrogen (H 2 ) measurements to quantitate carbohydrate absorption. Study of partially gastrectomized patients.

A technique was developed to quantitate the absorption of ingested carbohydrate by means of continuous measurements of pulmonary H(2) excretion. This technique is based on the observation that H(2) is produced in the colon when carbohydrate is fermented by colonic bacteria, and this H(2) is then excreted by the lungs. The quantitative relationship of pulmonary H(2) excretion to unabsorbed carbohydrate was studied in nine subjects. After ingestion of 6.5, 13, and 26 g of lactulose (a nonabsorbable disaccharide), H(2) excretion increased linearly, averaging (+/-1 SEM) 13+/-3.5, 23+/-7.2, and 49+/-7 ml per 2 hr. Because of consistent individual differences in H(2) excretion per gram of lactulose, the variability of this linear response was less in a given subject, with the H(2) excretion after 6.5 g and 26 g lactulose dosages averaging 55+/-4.2% and 214+/-16% of that observed after the 13 g dose. It was further demonstrated with fecal homogenates, as well as in rats after direct intracecal instillation of carbohydrate, that there was no significant difference in the rate of H(2) formation from lactulose as compared with the normally ingested sugars. Thus, a subject's H(2) excretion after a 13 g dose of lactulose can be used as a standard to convert H(2) excretion after ingestion of other carbohydrates into grams of carbohydrate not absorbed. Application of this technique to seven partially gastrectomized patients indicated all subjects malabsorbed a portion of a 100 g dose of glucose whereas six of seven completely absorbed a 25 g dose. Malabsorption of physiologic quantities of various carbohydrates was clearly demonstrated in one subject. This technique appears to provide quantitative information on carbohydrate malabsorption not readily obtained by presently available techniques.

Carbohydrate Metabolism↗
PubMed1972

Simultaneous study of the metabolic turnover and renal excretion of salivary amylase- 125 I and pancreatic amylase- 131 I in the baboon.

The metabolic turnover of salivary and pancreatic amylase was studied in the baboon, an animal with a serum amylase level and renal clearance of amylase similar to man. Purified amylase was electrolytically iodinated. Although iodinated and uniodinated amylase had similar gel filtration, electrophoretic, enzymatic, glycogen precipitation characteristics, the labeled enzyme was cleared less rapidly by the kidney than was the unlabeled material. However, urinary iodinated amylase which had been biologically screened by the kidney had a renal clearance and serum disappearance rate indistinguishable from unlabeled amylase and thus can serve as a tracer in metabolic turnover studies. Administration of a mixture of salivary amylase-(125)I and pancreatic amylase-(131)I made it possible to simultaneously measure the serum disappearance and renal clearance of these two isoenzymes. The metabolic clearance of both isoenzymes was extremely rapid with half-times of about 130 min. This rapid turnover of serum amylase probably accounts for the transient nature of serum amylase elevation which frequently occurs in pancreatitis. Pancreatic amylase-(131)I was consistently cleared more rapidly (mean clearance ratio: 1.8) by the kidney than was salivary amylase-(125)I. This more rapid renal clearance of pancreatic amylase may help to explain the disproportionate elevation of urinary amylase relative to serum amylase observed in pancreatitis.

Amylases↗
PubMed1971

Factors influencing pulmonary methane excretion in man. An indirect method of studying the in situ metabolism of the methane-producing colonic bacteria.

Measurements of pulmonary excretion of methane (CH(4)) were used to obtain information on the CH(4)-producing bacteria in man. Preliminary studies indicated that (a) all CH(4) excreted by man is produced by colonic bacteria, (b) there is no appreciable utilization of CH(4) by man, and (c) breath CH(4) can serve as a relatively accurate indicator of CH(4) production in the intestine. The rate of pulmonary CH(4) excretion varied enormously, ranging from undetectable (<5 x 10(-6) ml/min) to 0.66 ml/minute. In general, the CH(4) excretion rate for subjects was consistently very low (nonproducers) or relatively large (producers). 33.6% of the adult population were producers of CH(4). Whereas diet, age over 10 yr, and sex did not influence the rate of CH(4) production, some familial factor appeared to play an important role. 84% of siblings of CH(4) producers also were producers, while only 18% of the siblings of nonproducers were found to be CH(4) producers. This familial tendency appeared to be determined by early environmental rather than genetic factors. These studies of CH(4) excretion demonstrate that the exposure of individuals to intestinal bacterial metabolites may differ markedly and that these differences may be chronic and determined by familial factors.

ABO Blood-Group System↗
PubMed1971

Distribution, turnover, and mechanism of renal excretion of amylase in the baboon.

Pure amylase was isolated from pancreata and parotid glands of the baboon, an animal which has a serum amylase level and renal clearance of amylase (C(Am)) similar to man. After bolus injection, both pancreatic and salivary amylase rapidly disappeared from the serum in a monoexponential fashion with a mean serum half-time of approximately 83 min. Only about 24% of the amylase cleared from the serum appeared in the urine indicating that the majority of amylase was removed from the serum by an extraurinary mechanism. The C(Am) by the kidney was constant over a wide range of serum amylase levels and the ratio of C(Am)/C(In), which averaged 3.0%, was not influenced by mannitol diuresis. This suggests that the renal excretion of amylase results from glomerular filtration without appreciable tubular reabsorption. Pancreatic amylase was consistently cleared more rapidly by the kidney than was the baboon's endogenous amylase while salivary amylase was consistently cleared less rapidly than endogenous amylase. THE FINDINGS IN THIS STUDY PROVIDE INSIGHT INTO SEVERAL OF THE FOLLOWING CLINICALLY OBSERVED PHENOMENA: (a) the short serum half-time of amylase accounts for the transient nature of serum amylase elevations in pancreatitis; (b) the extra-urinary removal of amylase accounts for the maintenance of relatively normal amylase levels in uremia; and (c) the more rapid renal clearance of pancreatic amylase compared to salivary amylase may explain the disproportionate elevation of the urinary amylase excretion rate relative to the serum amylase level in acute pancreatitis.

Amylases↗