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Biomedical subjects

N C Turner

Publications and source records attributed to N C Turner.

51 records · Page 3Linked to original sources
PubMed1981

Histamine H1- and H2-receptors in the gastric vasculature of the rabbit.

The histamine receptors in the rabbit blood perfused gastric vasculature were analysed pharmacologically. Histamine elicited a monophasic increase in perfusion pressure which was antagonized by mepyramine and enhanced by metiamide. The maximum observed response was enhanced by metiamide to that produced by a specific H1-receptor agonist. It is concluded that the gastric vasculature responds to histamine with an H1-receptor mediated vasoconstriction and an H2-receptor mediated dilatation. In this preparation the H1-effect predominates in response to injection of histamine.

Animals↗
PubMed1981

Correction of flow resistances of plants measured from covered and exposed leaves.

The difference in water potential between an enclosed nontranspiring leaf and an adjacent exposed transpiring leaf, and the transpiration rate of a similarly exposed leaf, were used to calculate the change in hydraulic resistance of sorghum (Sorghum bicolor [L.] Moench) and sunflower (Helianthus annuus L.) leaves throughout the day and at various rates of transpiration. Since cotton (Gossypium hirsutum L.) leaves enclosed in aluminum foil alone had enclosed leaf water potentials about 0.06 megapascals lower than similar leaves enclosed in a polyethylene bag shielded with aluminum foil, the sorghum and sunflower leaves were enclosed in polyethylene bags shielded with aluminum foil. Enclosing the exposed leaf in a plastic sheath just prior to excision led to the water potential measured by the pressure chamber technique being 0.3 to 0.4 megapascals higher at rapid transpiration rates than in exposed leaves not sheathed just prior to excision. This error, previously shown to arise from rapid water loss after excision, led to an overestimation of the leaf hydraulic resistance in both species. Correction of the error reduced the resistance by 40 to 90% in irrigated sorghum and by about 40% in irrigated and unirrigated sunflower. After correction, the hydraulic resistances were still flow-dependent, but the dependency was markedly reduced in sorghum.

Journal Article↗
PubMed1981

Comparison of the agonist activity of impromidine (SK & F 92676) in anaesthetized rabbits and on the rabbit isolated fundic mucosa.

1 The actions of impromidine in vivo and in vitro have been compared on gastric acid secretion. In both preparations impromidine was more potent than histamine. 2 There were no statistically significant differences between the maximum responses to impromidine and histamine in vivo or in vitro although in vitro the maximum response to impromidine was only 67% of that to histamine. In vivo the maximum response to impromidine was significantly less than the maximum response to histamine plus mepyramine. 3 The results are discussed in the light of recent findings.

Animals↗
PubMed1981

The involvement of histamine receptor subtypes in gastric acid secretion and mucosal blood flow in the anaesthetized rabbit.

1. The involvement of histamine receptor subtypes in the gastric acid secretory and mucosal blood flow responses of the pentobarbitone-anaesthetized rabbit to histamine were investigated. 2. Histamine produced a dose-related increase in gastric acid secretion and mucosal blood flow. Maximal effects were stimulated further by mepyramine and inhibited by metiamide. Mepyramine had no effect on methacholine- or impromidine-stimulated gastric acid secretion or mucosal blood flow. 3. Impromidine also produced a dose-related increase in gastric acid secretion and mucosal blood flow. The maximal effect produced depended on the method of administration. The H1-receptor agonist 2-TEA inhibited impromidine-stimulated blood flow but not gastric acid secretion. 4. It is concluded that, in addition to stimulating acid secretion by an action on H2-receptors, histamine also exerts an inhibitory effect which is mediated by H1-receptors and that this inhibitory effect operates via reduced blood flow resulting from vasoconstriction in the stomach.

Animals↗
PubMed1975

Water uptake, diameter change, and nonlinear diffusion in tree stems.

A diffusion model for phloem swelling and contraction is proposed in which the rate of water movement changes markedly with moisture content. Good agreement between the actual swelling of the phloem of cotton stems and that predicted by the model was obtained. This result implies that water moves more readily into the phloem when it becomes wetter. This model also explains the lag of shrinkage of pine stems behind the water potential of the foliage and predicts that the lag is related to the thickness of the phloem.

Journal Article↗
PubMed1975

Influence of a Ceratocystis ulmi Toxin on Water Relations of Elm (Ulmus americana).

Water-soluble glycopeptides isolated from cultures of Ceratocystis ulmi have been reported to be toxins involved in Dutch elm disease. The influence of the glycopeptides on the water relations of Ulmus americana seedlings was tested by placing cut stems in glycopeptide preparations. After 4 hours in 200 micrograms per milliliter toxin the stem conductance of the seedlings was reduced by 79% and the leaf water potential was reduced by 3 bars to that at which the seedlings wilted, the stomata closed, and transpiration decreased. Decrease in stem conductance as the mode of action of the toxin was further confirmed by forcing toxin through the stem and petiole of elm and measuring the effects on stem conductance. High molecular weight dextrans were found to mimic the action of toxin on stem and petiole conductance, and their ability to do so was found to be correlated with their molecular weight. As low as 4 micrograms of toxin or dextrans were found to measurably decrease the stem and petiole conductance of elms. Disruption of the water-conducting system of elms and other plants by small quantities of high molecular weight compounds may be a factor in diseases with wilting symptoms.

Journal Article↗
PubMed1975

Changes in Alfalfa Stem Conductance Induced by Corynebacterium insidiosum Toxin.

A toxin involved in bacterial wilt of alfalfa (Medicago sativa L.) has been isolated from cultures of the pathogen, Corynebacterium insidiosum, as well as from diseased plants (S. M. Ries and G. A. Strobel. 1972. Physiological Plant Pathology 2: 133-142). The influence of this toxin, a glycopeptide with a molecular weight of 5 x 10(6), on the water relations of alfalfa was examined. It was found that very small amounts of the toxin (2 mug) significantly reduced stem conductance through 15-cm long stems. This decrease in stem conductance caused by the toxin best explains the rapid decrease in transpiration and stomatal conductance and the resultant wilting after alfalfa cuttings have been in 200 mug ml(-1) toxin for 2 hours. Membrane damage resulting in water leakage was ruled out as a factor in the wilting during the 2-hour period. It is postulated that the toxin acts by interfering with water movement through pit membranes.

Journal Article↗
PubMed1975

Concurrent comparisons of stomatal behavior, water status, and evaporation of maize in soil at high or low water potential.

Concurrent measurements of evaporation, leaf conductance, irradiance, leaf water potential, and osmotic potential of maize (Zea mays L. cv. Pa602A) in soil at either high or low soil water potential were compared at several hours on two consecutive days in July. Hourly evaporation, measured on two weighing lysimeters, was similar until 1000 hours Eastern Standard Time, but thereafter evaporation from the maize in the dry soil was always less than that in the wet soil; before noon it was 62% and by midafternoon, only 35% of that in the wet soil. The leaf water potential, measured with a pressure chamber, was between -1.2 and -2.5 bars and between -6.8 and -8 bars at sunrise (about 0530 hours Eastern Standard Time) in the plants in the wet and dry soil, respectively, but decreased quickly to between -8 and -13 bars in the plants in the wet soil and to less than -15 bars in the plants in the dry soil by 1100 to 1230 hours Eastern Standard Time. At this time, the leaf conductance of all leaves was less than 0.1 cm sec(-1) in the maize in the dry soil, whereas the conductance was 0.3 to 0.4 cm sec(-1) in the leaves near the top of the canopy in the wet soil. The osmotic potential, measured with a vapor pressure osmometer, also decreased during the morning but to a smaller degree than leaf water potential, so that by 1100 to 1230 hours Eastern Standard Time the leaf turgor potential was 1 to 2 bars in all plants. Thereafter, leaf turgor potential increased, particularly in the plants in soil at a high water potential, whereas leaf water potential continued to decrease even in the maize leaves with partly closed stomata. Evidently maize can have values of leaf conductance differing 3- to 4- fold at the same leaf turgor potential, which suggests that stomata do not respond primarily to bulk leaf turgor potential. Evidence for some osmotic adjustment in the plants at low soil water potential is presented. Although the degree of stomatal closure in the maize in dry soil did not prevent further development of stress, it did decrease evaporation in proportion to the decrease in canopy conductance.

Journal Article↗
PubMed1974

Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions: II. At Low Soil Water Potential.

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), leaf resistance (r(1)), leaf water potential (psi), osmotic potential (pi), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. cv. Pa602A), sorghum (Sorghum bicolor [L.] Moench cv. RS 610), and tobacco (Nicotiana tabacum L. cv. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were low. The r(1), measured with a ventilated diffusion porometer, of the leaves in the upper canopy decreased temporarily after sunrise [ approximately 0530 hours Eastern Standard Time] as I increased, but then r(1) increased again between 0700 and 0830 hr Eastern Standard Time as the psi, measured with a pressure chamber, decreased rapidly from the values of -7, -4 and -6 bars at sunrise to minimal values of -18, -22 and -15 bars near midday in the maize, sorghum, and tobacco, respectively. The pi, measured with a vapor pressure osmometer, also decreased after sunrise, but not to the same degree as the decrease in psi, so that a P of zero was reached in some leaves between 0730 and 0800 hours. The lower (more negative) pi of leaves in the upper canopy than those in the lower canopy gave the upper leaves a higher P at a given psi than the lower leaves in all three species; leaves at intermediate heights had an intermediate P. This difference between leaves at the three heights in the canopy was maintained at all values of psi. The r(1) remained unchanged over a wide range of P and then increased markedly at a P of 2 bars in maize, -1 bar in sorghum, and near zero P in tobacco: r(1) also remained constant until psi decreased to -17, -20, and -13 bars in leaves at intermediate heights in maize, sorghum, and tobacco, respectively. In all three species r(1) of leaves in the upper canopy increased at more negative values of psi than those at the base of the canopy, and in tobacco, leaves in the upper canopy wilted at more negative values of psi than those in the lower canopy.

Journal Article↗
PubMed1973

Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions: I. At High Soil Water Potential.

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), stomatal resistance (r(s)), leaf water potential (psi), osmotic potential (pi), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. var. Pa 602A), sorghum (Sorghum bicolor [L.] Moench var. RS610), and tobacco (Nicotiana tabacum L. var. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were high. In all three crops the r(s), measured with a ventilated diffusion porometer, the psi, measured with the pressure chamber, the pi, measured with a vapor pressure osmometer, and the calculated P, decreased from sunrise to reach minimum values near midday and then increased again in the afternoon. The diurnal range of all the variables was greater for leaves in the upper canopy than for those in the lower canopy. P was observed to decrease with decreasing psi, but never became zero. Sorghum had a higher P at a psi of, say -10 bars, than did maize, and maize had a higher P than tobacco at the same psi. Moreover, at the same psi the upper leaves in all canopies had a higher P than the lower leaves. When compared at high irradiances, r(s) did not increase as psi declined to -13, -15, and -10 bars or as P declined to 0.3, 3.5, and 1.2 bars in maize, sorghum, and tobacco, respectively. The relation between r(s) and I in the upper, nonsenescent leaves of all three crops fits a hyperbolic curve, but the response varied with species and leaf senescence. The adaxial and abaxial epidermises had the same response of r(s) to I in maize and sorghum, whereas in tobacco the adaxial epidermis had a higher r(s) than the abaxial epidermis at all values of I. At equal values of I, tobacco had the lowest leaf resistance (r(l)) and maize had the highest r(l). Senescent maize leaves had nonfunctional stomata, whereas the lowermost sorghum leaves had higher stomatal resistances on average than the other leaves.

Journal Article↗
PubMed1970

Water potential gradients in field tobacco.

A pressure chamber was used to establish the vertical gradients of leaf water potential (Psi(Leaf)) and stem water potential (Psi(Stem)) in field-grown tobacco (Nicotiana tabacum L. var. Havanna seed 211) at three different times of day. Leaves enclosed in polyethylene bags and aluminum foil the previous afternoon and left to equilibrate overnight were used to determine Psi(Stem). The greatest difference between Psi(Leaf) and Psi(Stem) occurred in the upper part of the plant at 1100 hours Eastern Standard Time and was 5.5 bars. The largest vertical gradient in Psi(Stem) occurred at 1300 hours. The soil water potential (Psi(Soil)), extrapolated from the potential of leaves on a completely enclosed plant, was higher than -1 bar. The vertical gradient in Psi(Stem) and the difference between Psi(Leaf) and Psi(Stem) showed the existence of a resistance to water movement within the stem (r(stem)) and a further resistance between the stem and leaf (r(petiole)). The r(petiole) and root resistance (r(root)) were estimated to be 931 and 102 bars seconds per cubic centimeter, respectively. The r(stem) was low (94 bars seconds per cubic centimeter) at 1100 hours but increased to 689 bars seconds per cubic centimeter at 1300 hours.

Journal Article↗
PubMed1968

Effects of changing stomatal width in a red pine forest on soil water content, leaf water potential, bole diameter, and growth.

Spraying a 16 meter tall stand of red pine (Pinus resinosa Ait.) with 10(-3)m phenylmercuric acetate in early June and again in mid-July resulted in the water use between June 1 and October 25 being reduced by almost 10%. It was demonstrated that this was caused by an increase in the leaf resistance with partial stomatal closure, which reduced absolute water potential in the needles by 1 to 3 bars in the middle of the day. Smaller demands were made upon the reserves of water in the bole of the tree as shown by the smaller bole contraction in the treated trees. Although needle length and dry weight were unaffected by the spray, radial growth was reduced by approximately 32%. The dependence of leaf resistance on light intensity is shown, and its independence from leaf water potential discussed.

Journal Article↗