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Biomedical subjects

M A Simmons

Publications and source records attributed to M A Simmons.

At least 19 recordsLinked to original sources
PubMed1992

Intracellular guanosine-5'-O-(2-thiodiphosphate) alters the dynamics of receptor-mediated responses in bullfrog sympathetic neurons.

The mechanism by which intracellularly applied guanosine-5'-O-(2-thiodiphosphate) alters responses to chicken II luteinizing hormone-releasing hormone, muscarine, and substance P in bullfrog sympathetic neurons was examined. Whole-cell recordings were made from enzymatically dissociated single neurons. Guanosine-5'-O-(2-thiodiphosphate) was applied intracellularly by adding it to the pipette solution with fixed amounts of GTP. Guanosine-5'-O-(2-thiodiphosphate) did not affect the proportion of cells that responded to any of the agonists. Guanosine-5'-O-(2-thiodiphosphate) decreased the amplitude of the responses to submaximal concentrations of agonist. At maximal concentrations of agonist, guanosine-5'-O-(2-thiodiphosphate) did not decrease the response to the first application of agonist; however, with guanosine-5'-O-(2-thiodiphosphate) intracellularly, successive responses to maximal concentrations of agonist were decreased in amplitude and increased in time course. Intracellular guanosine-5'-O-(2-thiodiphosphate) did not accelerate the rate or magnitude of desensitization to substance P. A kinetic model of receptor-guanine nucleotide-binding protein (G protein) coupling predicts that a decrease in the available G protein pool should decrease both the magnitude and the time course of the build-up of active G proteins. The results are consistent with the hypothesis that guanosine-5'-O-(2-thiodiphosphate) binds tightly to G proteins, thereby effectively decreasing the available G protein pool with repeated agonist applications.

Animals↗
PubMed1991

Muscarinic suppression of the M-current is mediated by a rise in internal Ca2+ concentration.

The role of intracellular Ca2+ in the muscarinic suppression of M-current was examined. Intracellular injection of Ca2+ buffer into cells in the intact ganglion reduced the response to muscarinic agonist. In similar experiments on isolated cells, Ca2+ buffer was introduced into the cytoplasm using a perfused recording pipette. Ca2+ buffer (20 mM) with the free Ca2+ concentration set to normal resting levels produced a reversible reduction of the muscarinic response. In a second line of investigation, it was found that pharmacological procedures designed to deplete internal stores of Ca2+ produced a decrease in the muscarinic response. These results, taken together with previous work, support the hypothesis that the muscarinic suppression of M-current is mediated by the release of Ca2+ from intracellular stores.

Animals↗
PubMed1991

Selectivity of the effects of guanosine-5'-O-(2-thiodiphosphate) on agonist inhibition of the M-current in amphibian sympathetic neurons.

In bullfrog sympathetic neurons, luteinizing hormone-releasing hormone, muscarine, and substance P act as agonists at specific membrane receptors to decrease a potassium current, IM. The receptors are coupled to guanine nucleotide-binding proteins (G-proteins). Whole-cell recordings of IM were made from isolated bullfrog sympathetic neurons to examine the effects of intracellularly applied guanosine-5'-O-(2-thiodiphosphate) (GDP beta S) on agonist inhibition of IM. Successive responses to a given agonist were decreased in the presence of GDP beta s. Subsequent responses to the other agonists were then measured to determine the degree of overlap of the effect of GDP beta S for the different agonists. GDP beta S selectively inhibited successive responses to one agonist such that a subsequent application of a different agonist was still effective. If GDP beta S acts at the level of the G-protein, this suggests that each receptor is coupled to a separate population of G-proteins. Alternatively, GDP beta S may act at the receptor level to block receptor coupling to IM.

Animals↗
PubMed1990

Desensitization of the inhibition of the M-current in sympathetic neurons: effects of ATP analogs, polyanions, and multiple agonist applications.

Desensitization occurs when the response to a neurotransmitter receptor agonist wanes in the continued presence of agonist. In amphibian sympathetic neurons, both muscarinic and peptidergic receptor agonists inhibit a K+ current, the M-current (IM), and this inhibition desensitizes. We have studied the desensitization to substance P (SP) by whole-cell recordings from dissociated sympathetic neurons from bullfrogs. When ATP in the recording pipette was replaced with AMP-PNP, SP still inhibited IM, but no desensitization was observed, indicating that ATP hydrolysis is required for desensitization. Desensitization inhibitors of beta-adrenergic receptors did not block desensitization to SP. When a low dose of muscarine sufficient to inhibit IM, but not to elicit desensitization, was applied simultaneously with a desensitizing dose of SP, IM remained depressed and did not desensitize. Thus, there may be separate systems controlling desensitization for different agonists, or the enzyme(s) involved is "compartmentalized."

Adenosine Triphosphate↗
PubMed1990

Short-term bioconcentration studies of Np in freshwater biota.

Short-term laboratory exposures were conducted to determine the potential accumulation of Np in aquatic organisms. Concentration factors were highest in green algae. Daphnia magna, a filter-feeding crustacean, accumulated Np at levels one order of magnitude greater than the amphipod Gammarus sp., an omnivorous substrate feeder. Accumulation of Np in juvenile rainbow trout (Oncorhynchus mykiss) was highest in carcass (generally greater than 78% of the total body burden) and lowest in fillets. Recommended concentration factors for Np, based on fresh weight, were 300 for green algae, 100 for filter-feeding invertebrates, for nonfilter-feeding invertebrates, 10 for whole fish, and one for fish flesh.

Animals↗
PubMed1989

An endogenous 'hypertensive factor' enhances the voltage-dependent calcium current.

The effects of an immunoaffinity-purified putative endogenous hypertensive factor (HF) on voltage-dependent calcium current in frog cardiac myocytes were assessed. In 9 out of 10 cells, HF reversibly increased the peak amplitude of the calcium current. HF increased peak calcium current density at -5 mV from a control level of 1.8 +/- 1.3 pA/pF (mean +/- SD) to 4.4 +/- 2.0 pA/pF. HF shifted the peak of the calcium current-voltage relationship in the hyperpolarizing direction. HF shifted the voltage dependence of the inactivation of the calcium current to more negative potentials with prepulses from -80 to 0 mV, but the inactivation was not affected with prepulses more positive than 0 mV. Modulation of the voltage-dependent calcium current by HF may be the mechanism underlying its pressor effects.

Action Potentials↗
PubMed1989

Fate of milk calcium in the gastrointestinal tract of suckling rats.

In the rat, as in many other mammals, the majority of milk Ca is associated with casein micelles. Because luminal proteolysis is immature in the suckling rat, we investigated the hypothesis that milk Ca remains in the macromolecular form as it traverses the stomach and upper small intestine. When intestinal contents of rat pups were examined 1 h after gavage with 45Ca-milk, 98% of the 45Ca was found to be in a soluble form. Surprisingly, chromatography on Sephadex G-10 showed it to have a molecular weight in the range of that of Ca2+. Analysis of gastric contents showed the stomach to be the initial site of release of Ca from the casein micelles. Although only 45% of the 45Ca was in a soluble form in stomach contents, 100% of this soluble 45Ca behaved as Ca2+. Comparison with the fate of 32P from milk confirmed the suggestion that most of the Ca is released as Ca2+. In vitro studies suggested that this release is the result of simple dilution or the combined action of intrinsic milk enzymes and dilution. Because milk Ca appears to enter the small intestine as Ca2+, we presume its mechanism of absorption in suckling animals is the same as that already established using simple salts of Ca. Nevertheless, the slow release from the bound form may be important in establishing the physiological pattern of Ca absorption in the infant. Implications of this for the supplementation of milks for low-birth-weight human infants is discussed.

Animals↗
PubMed1989

A randomized trial to develop criteria for administering erythrocyte transfusions to anemic preterm infants 1 to 3 months of age.

A randomized trial of erythrocyte transfusion vs no transfusion was performed in 16 preterm infants 1 to 3 months old with hematocrits of less than or equal to 0.29 L/L. To determine which (if any) such patients definitely benefit from transfusion, an analysis of outcome variables was performed. Factors that prospectively identified patients who would benefit from transfusions included a heart rate of greater than 152 beats per minute (P less than .01), apnea/bradycardia (heart rate less than 90/min) requiring intervention to increase the heart rate (P less than .01), and a blood lactate level above the reference range (P less than .02). Additional investigations were performed to determine the cause of the low hematocrits in the study patients. All had diminished, rather than accelerated, erythropoiesis. However, neither the anemia of chronic disorders nor iron deficiency anemia contributed to the diminished erythropoiesis. In all cases, serum erythropoietin levels were below the predicted range (P less than .001). Thus, at least some preterm infants aged 1 to 3 months with hematocrits less than or equal to 0.29 L/L definitely derive benefit from erythrocyte transfusion. The presence of tachycardia, apnea/bradycardia, or an elevated blood lactate may prospectively identify such patients.

Anemia, Neonatal↗
PubMed1988

Responsiveness to recombinant human erythropoietin of marrow erythroid progenitors from infants with the "anemia of prematurity".

We used cells from marrow aspirations that had been performed on 10 infants with the "anemia of prematurity" and tested the responsiveness of their erythroid colony-forming units (CFU-E) to recombinant human erythropoietin. For comparison, we also tested marrow-derived CFU-E from five healthy adults, and circulating CFU-E from cord blood of five healthy neonates. CFU-E from the anemic infants had a 50% maximal response at 0.073 +/- 0.024 U erythropoietin per milliliter (mean +/- SD). They were therefore at least as responsive as were CFU-E from adults, which displayed a 50% maximal response at 0.118 +/- 0.076 U/ml, and as were circulating CFU-E of cord blood origin, which had a 50% maximal response at 0.109 +/- 0.047 U/ml. Because CFU-E from infants with the "anemia of prematurity" appeared highly sensitive to erythropoietin in vitro, we propose that its administration to these patients would likely result in a significant increase in erythrocyte production in vivo.

Anemia, Neonatal↗
PubMed1988

Role of phosphodiesterase in regulation of calcium current in isolated cardiac myocytes.

It has previously been shown that intracellular perfusion of isolated cardiac myocytes with cGMP reduces the amplitude of the trans-sarcolemmal calcium current (ICa) elevated by cAMP-dependent mechanisms. To test the hypothesis that cGMP acts by stimulating a cyclic nucleotide phosphodiesterase (PDE) activity, PDE activity and the effects of methylisobutylxanthine (MIX), a PDE inhibitor, on ICa were examined in cardiomyocytes dissociated from frog ventricle. PDE activity was determined by measuring hydrolysis of [33P]cAMP in subcellular fractions. Using 100 microM cAMP as substrate, approximately 50% of the PDE activity was found in the 20,000 x g particulate fraction. Basal activity in this fraction had a Vmax of 15.4 nmol [corrected] of cAMP hydrolyzed/min/mg of protein and a Km of 113 microM cAMP. The PDE activity of the particulate fraction was stimulated significantly by cGMP. Half-maximal stimulation was observed with 1.1 microM cGMP. This value is virtually identical to the value for the concentration of intracellular cGMP that produced a half-maximal reduction of cAMP-elevated ICa in electrophysiological experiments. The cGMP-stimulated PDE activity had a Vmax of 9.5 nmol/min/mg [corrected] and a Km of 12.3 microM cAMP. MIX (100 microM) selectively inhibited the cGMP-stimulated PDE activity (IC50 = 20 microM). To determine whether PDEs modulate the amplitude of ICa, the effects of MIX were examined on basal ICa and cAMP-elevated ICa. MIX produced small increases in the basal ICa and increased ICa in the presence of 1 microM intracellular cAMP. MIX at 100 microM potentiated the effects of submaximal doses of isoproterenol and shifted the dose-response curve for cAMP to the left but did not affect the dose-response curve for 8-bromo-cAMP. MIX reversed the effect of cGMP on the cAMP-elevated ICa. We conclude that cyclic nucleotide PDEs play an important role in modulating the cardiac calcium current. The hypothesis that cGMP inhibits the cAMP-elevated ICa by activating a PDE is supported by the finding that MIX inhibited both the cGMP-stimulated PDE activity and the effect of cGMP on ICa at similar concentrations.

1-Methyl-3-isobutylxanthine↗
PubMed1987

A quantitative analysis of the acetylcholine-activated potassium current in single cells from frog atrium.

The K+ current activated by acetylcholine (ACh) in single cells from the frog atrium was analyzed using the whole-cell patch clamp technique. The ACh current was analyzed quantitatively by subtracting the currents elicited in response to voltage steps in the presence and absence of a steady bath-application of 1 microM ACh. The net ACh currents were voltage- and time-dependent. With depolarizing jumps, the ACh-activated current declined from an instantaneous peak to a new steady level. With hyperpolarizations, the instantaneous current change was followed by a time-dependent increase in current. The current relaxations were well fitted by the sum of two exponentials with time constants of approximately 20 ms and approximately 300 ms at -120 mV. Both time constants decreased with depolarization. The current-voltage relationship inwardly-rectified. This inward rectification was due both to a decrease in the single channel conductance and a decrease in the number of open channels with depolarization. The ACh-activated K+ current differs from the background K+ current in several respects. The kinetics of the background K+ current are much more rapid and the background K+ channel passes much less current in the outward direction than the ACh channel.

Acetylcholine↗
PubMed1987

Comparison of effects of acetylcholine on calcium and potassium currents in frog atrium and ventricle.

1. Ca2+ and K+ currents were measured in single atrial and ventricular myocytes from frog heart with the whole-cell patch-clamp technique. 2. K+ currents were blocked with intra- and extracellular Cs+ and the fast Na+ current was blocked with tetrodotoxin (TTX). The Ca2+ current (ICa) was evoked by a depolarizing pulse from -80 to 0 mV. ICa was larger in ventricular (3.4 +/- 2.5 microA/cm2) than atrial (1.6 +/- 2.5 microA/cm2) myocytes. 3. Acetylcholine (ACh) had no effect on basal ICa when K+ currents were blocked with Cs+ or Ba2+. Isoprenaline increased ICa and ACh reduced the isoprenaline-stimulated current to basal levels. 4. In contrast, when K+ currents were not blocked, ACh reduced the net inward current and increased the outward current at the end of the depolarizing pulse. The outward current was studied in the presence of Cd2+ to block ICa. The steady-state current-voltage relationship inwardly rectified and reversed near the K+ reversal potential (EK). The magnitude of the steady-state ACh-activated K+ current at 0 mV was 1.0 +/- 0.7 microA/cm2 in ventricular cells and 3.67 +/- 1.7 microA/cm2 in atrial cells. 5. With depolarization, the outward current increased instantaneously and then decreased to a new steady level. The first phase of the decay occurred with a time constant similar to that of the activation of ICa. The Cd2+-sensitive current (corresponding to ICa) was obtained by subtracting currents in the presence and absence of Cd2+. The Cd2+-sensitive current was not affected by ACh. 6. The apparent effect of ACh on basal ICa can be explained quantitatively by activation of a time-dependent K+ current by ACh that contaminates ICa.

Acetylcholine↗
PubMed1986

A time-dependent and voltage-sensitive K+ current in single cells from frog atrium.

A quantitative description of the time-dependent and voltage-sensitive outward currents in heart has been hampered by the complications inherent to the multicellular preparations previously used. We have used the whole-cell patch-clamp technique to record the delayed outward K+ current, IK, in single cells dissociated from frog atrium. Na+ currents were blocked with tetrodotoxin and Ca2+ currents with Mn2+ or Cd2+. After depolarizations from -50 mV to potentials positive to -30 mV, a time-dependent outward current was observed. This current has been characterized according to its steady state activation, kinetics, and ion transfer function. The current is well described as a single Hodgkin-Huxley conductance. The deactivation of the current is a single exponential. Activation of the current is sigmoid and is fitted well by raising the activation variable to the second power. The reversal potential of IK is near EK and shifts by 57 mV/10-fold change in [K+]o. This suggests that the current is carried selectively by K ions. The threshold for activation is near -30 mV. IK is maximally activated positive to +20 mV and shows no inactivation. The fully activated current-voltage relationship is linear between -110 and +50 mV. Neither Ba2+ (250 microM) nor Cd2+ (100 microM) affects IK.

Action Potentials↗
PubMed1985

Synaptic transmission in the rabbit inferior mesenteric ganglion.

Electrical properties, cholinergic neurotransmission and non-cholinergic neurotransmission in the rabbit inferior mesenteric ganglion (IMG) in vitro were examined with intracellular recording techniques. A single ganglionic neuron received an average of 42 nicotinic cholinergic synaptic inputs. An atropine-sensitive slow excitatory postsynaptic potential not followed by a non-cholinergic late slow excitatory postsynaptic potential (LS-EPSP) was observed in 7% of the cells. In 63% of the cells a LS-EPSP insensitive to antagonism of nicotinic and muscarinic receptors was observed following repetitive nerve stimulation. The involvement of substance P (SP) in the genesis of the LS-EPSP was tested by applications of SP, applications of SP antagonists and applications of capsaicin. Neither SP, SP antagonists nor capsaicin affected the LS-EPSP. These findings distinguish the LS-EPSP in the rabbit IMG from its counterpart in the guinea pig IMG where SP has been proposed as the mediator of the LS-EPSP. A late slow inhibitory postsynaptic potential was observed in 13% of the cells. This hyperpolarization followed repetitive nerve stimulation and was insensitive to blockade of cholinergic receptors. There is a marked convergence of subthreshold fast excitatory postsynaptic potentials (F-EPSPs) of both central and peripheral origin onto these cells. The LS-EPSP could provide a mechanism for increasing the likelihood of temporal and/or spatial summation of these fast synaptic inputs, thereby increasing the probability of action potential generation in the ganglion cells.

Action Potentials↗
PubMed1979

Coagulation studies in extremely premature infants.

Evidence of developmental evolution of coagulation can be seen when the studies of 10 thriving extremely premature (EPT) infants are compared to normal full-term (FT) infants. The prothrombin time, partial thromboplastin time, and thrombin time all became shorter with increasing gestational age. Fibrinogen levels and platelet counts appear to be comparable to term infant and adult levels. Fibrin degradation products (FDP) of 10 micrograms/ml or less were found in the thriving EPT infants. When compared to healthy full-term infants, there is a definite gestational dependency of anti-thrombin III levels. Factors II and VII appear to be related to intrauterine maturation after the age of viability (24 wk), but factor VII-X complex does not. The contact factors XI, XII, high molecular weight kininogen (Fitzgerald factor), and prekallikrein (Fletcher factor) are all markedly decreased in thriving EPT infants. The mean factor V level is lower than that found in FT infants. This study confirms a gestational age dependency of factor VIII activity. The ratio of factor VIII antigen to factor VIII clotting activity is increased (2.8 vs 1.01 in FT and adults). Thriving small for gestational age (SGA) infants had coagulation studies which were not statistically different from those of thriving EPT infants. The coagulation changes which occurred in severely ill EPT were mainly in the factors which decrease during intravascular coagulation (factors I, V, and VIII). The present study suggests that because of the high antigen to activity ratio seen in thriving EPT infants, a dysfunctional or fetal factor VIII may have been produced. However, the further elevation of this ratio in the severely ill EPT infants is in keeping with a pathologic proteolysis or increased endothelial release of factor VIII antigen.

Blood Coagulation Factors↗
PubMed1979

Metabolic requirements for fetal growth.

Table 1 outlines a metabolic balance sheet for the sheep fetus. It is clear that maternal substrate concentrations as well as placental function are important in assuring the provision of adequate substrate to meet fetal metabolic and growth requirements. It is intriguing that the fetus appears to use substrates not usually regarded as important in extrauterine diets (lactate) and to use substrates for catabolic purposes normally thought to be primarily anabolic substrates (amino acids). This information emphasizes the hazards of extrapolating metabolic and nutritional patterns seen in extrauterine life in reaching conclusions concerning the fetus. It likewise emphasizes the importance of ongoing studies in maternal and fetal nutrition and metabolism.

Animals↗
PubMed1979

Results of the intensive perinatal management of very-low-birth-weight infants (501 to 1,500 grams).

In January 1975 at the University of Colorado Medical Center, a program of intensive intrapartum and neonatal care went into effect for all infants with expected birth weights of over 600 gm. Data are presented on the 187 infants weighing 501 to 1,000 gm born in 1975 to 1976. The 70 infants weighing 501 to 1,000 gm had a perinatal mortality of 65% and a neonatal mortality of 55%. The perinatal mortality of the 117 infants weighing 1,001 to 1,500 gm was 25% and the neonatal mortality 20%. Among the 501- to 1,000-gm infants, cesarean section for delivery of abnormal presentations resulted in a lower perinatal mortality than did vaginal delivery. Apgar scores were predictive of an improved chance of survival, but scores of three or less even at five minutes were associated with a 25% survival rate. Of those infants who did not survive the neonatal period, over 70% had died by 48 hours of life. These results were achieved without the use of beta-mimetic tocolytic agents to inhibit labor or long-acting corticosteroids to enhance pulmonary maturation. The improved survival of the infants weighing 1,500 gm or less when compared with infants of similar weights in preceding years is attributed to more intensive perinatal management of these mothers and their very-low-birth-weight infants.

Birth Weight↗