Model of calcium-induced calcium release mechanism in cardiac cells

A model with which to elucidate the mechanism of Ca(2+) release from, and Ca(2+) loading in the sarcoplasmic reticulum (SR) by Ca(2+) current (I Ca) in cardiac cells is proposed. The SR is assumed to be comprised of three functional subcompartments: (1) the main calcium store (MCS), which contains most of the calcium (both free and bound); (2) the releasable terminal (RT), which contains the calcium readily available for release; and (3) the longitudinal network of the SR (LSR), which sequesters and the transfers the sarcoplasmic calcium to the RT. A rapid increase of the Ca(2+) concentration at the outer surface of the SR (Cae) due to the fast component of I(Ca) activates and inactivates this surface, inducing the release of Ca(2+) from the RT to the sarcoplasmic space. The RT in turn is further activated and inactivated by a increase in the concentration of sarcoplasmic Ca(2+). The Ca(2+) in the sarcoplasmic space is then sequestered by the LSR, leading to the reactivation of the RT. Further increase of Cae due to the slow component of I(Ca) enhances the entry of Ca(2+) into the MCS to be bound by the binding substance. The free Ca(2+) released from the Ca-binding substance complex is transferred to the RT for subsequent release. The activation, inactivation and reactivation are Ca(2+)-mediated and time-dependent. The proposed model yields simulation of the many events qualitatively similar to those observed experimentally in skinned cardiac cells.

Observation of large-scale density cavities and parametric-decay instabilities in the high-altitude discrete auroral ionosphere under pulsed electromagnetic radiation

A large density cavity that measured 2000 km across and 500 km in height was observed by DEMETER and Formosat/COSMIC satellites in temporal and spatial relation to a new mode of propagation of electromagnetic (em) pulses between discrete magnetic field-aligned auroral plasmas to high altitudes. Recorded positive plasma potential from satellite probes is consistent with the expulsion of electrons in the creation of density cavities. High-frequency decay spectra support the concept of parametric instabilities fed by free energy sources.

Do-not-resuscitate decision: the attitudes of medical and non-medical students

OBJECTIVES

To study the attitudes of both medical and non-medical students towards the do-not-resuscitate (DNR) decision in a university in Hong Kong, and the factors affecting their attitudes.

METHODS

A questionnaire-based survey conducted in the campus of a university in Hong Kong. Preferences and priorities of participants on cardiopulmonary resuscitation in various situations and case scenarios, experience of death and dying, prior knowledge of DNR and basic demographic data were evaluated.

RESULTS

A total of 766 students participated in the study. There were statistically significant differences in their DNR decisions in various situations between medical and non-medical students, clinical and preclinical students, and between students who had previously experienced death and dying and those who had not. A prior knowledge of DNR significantly affected DNR decision, although 66.4% of non-medical students and 18.7% of medical students had never heard of DNR. 74% of participants from both medical and non-medical fields considered the patient's own wish as the most important factor that the healthcare team should consider when making DNR decisions. Family wishes might not be decisive on the choice of DNR.

CONCLUSIONS

Students in medical and non-medical fields held different views on DNR. A majority of participants considered the patient's own wish as most important in DNR decisions. Family wishes were considered less important than the patient's own wishes.

Direct acceleration of solid-density plasma bunch by ultraintense laser

The interaction of a petawatt laser with a small solid-density plasma bunch is studied by particle-in-cell simulation. It is shown that when irradiated by a laser of intensity >10(21) W/cm2, a dense plasma bunch of micrometer size can be efficiently accelerated. The kinetic energy of the ions in the high-density region of the plasma bunch can exceed ten MeV at a density in the 10(23)-cm(-3) level. Having a flux density orders of magnitude higher than that of the traditional charged-particle pulses, the laser-accelerated plasma bunch can have a wide range of applications. In particular, such a dense energetic plasma bunch impinging on the compressed fuel in inertial fusion can significantly enhance the nuclear-reaction cross section and is thus a promising alternative for fast ignition.

A method for measuring systolic and diastolic microcirculatory red cell flux within the canine myocardium

BACKGROUND

Knowledge of the patterns of movement of red cells during the cardiac cycle in the microcirculation within the contracting myocardium is largely unknown. We describe a method of making such measurements in the canine myocardium using the technique of laser Doppler velocimetry.

METHODS

A lensed 100 microm fiber-optic probe was inserted into the beating myocardium at various sites. Using an ultra-stable laser and achieving measurement stability by heterodyning the laser light and reflected light from the tissue, it was possible to obtain a stable high quality measurement of predominately red cell movement in the microcirculation.

RESULTS

Unique regional patterns of red cell movement within the myocardium were observed. Epicardial flux was continuous with peaks while endocardial flux was predominately diastolic. Stopping flow in the epicardial artery for 5-6 s demonstrated that red cell movement continues in the microcirculation with some reduction followed by a delayed reactive hyperemia. Modeling demonstrates an important role for the small coronary veins in control of microcirculatory red cell movement.

CONCLUSIONS

It is possible using laser Doppler velocimetry to measure red blood cell flux in the beating canine myocardium. Such measurements demonstrate a high degree of complexity which is not reflected in epicardial coronary arterial or venous flow.

Modulation of a presynaptic hyperpolarization-activated cationic current (I(h)) at an excitatory synaptic terminal in the rat auditory brainstem

1. A hyperpolarization-activated non-specific cation current, I(h), was examined in bushy cell bodies and their giant presynaptic terminals (calyx of Held). Whole-cell patch clamp recordings were made using an in vitro brain slice preparation of the cochlear nucleus and the superior olivary complex. The aim was to characterise I(h) in identified cell bodies and synaptic terminals, to examine modulation by presynaptic cAMP and to test for modulatory effects of I(h) activation on synaptic transmission. 2. Presynaptic I(h) was activated by hyperpolarizing voltage-steps, with half-activation (V(1/2)) at -94 mV. Activation time constants were voltage dependent, showing an e-fold acceleration for hyperpolarizations of -32 mV (time constant of 78 ms at -130 mV). The reversal potential of I(h) was -29 mV. It was blocked by external perfusion of 1 mM CsCl but was unaffected by BaCl(2). 3. Application of internal cAMP shifted the activation curve to more positive potentials, giving a V(1/2) of -74 mV; hence around half of the current was activated at resting membrane potentials. This shift in half-activation was mimicked by external perfusion of a membrane-permeant analogue, 8-bromo-cAMP. 4. The bushy cell body I(h) showed similar properties to those of the synaptic terminal; V(1/2) was -94 mV and the reversal potential was -33 mV. Somatic I(h) was blocked by CsCl (1 mM) and was partially sensitive to BaCl(2). Somatic I(h) current density increased with postnatal age from 5 to 16 days old, suggesting that I(h) is functionally relevant during maturation of the auditory pathway. 5. The function of I(h) in regulating presynaptic excitability is subtle. I(h) had little influence on EPSC amplitude at the calyx of Held, but may be associated with propagation of the action potential at branch points. Presynaptic I(h) shares properties with both HCN1 and HCN2 recombinant channel subunits, in that it gates relatively rapidly and is modulated by internal cAMP.

Findings in women with postmenopausal bleeding investigated with hysteroscopy

Most reports on histopathological findings of postmenopausal bleeding were published before hysteroscopy was widely used. Recent studies have shown that hysteroscopic examination is better than dilatation and curettage alone in detecting intrauterine pathology. The aim of this study is to assess histopathological findings of patients presented with postmenopausal bleeding after incorporation of hysteroscopic examination. Between 1 January 1996 and 31 December 1996, a total of 199 consecutive patients presenting with postmenopausal bleeding to a regional hospital were reviewed. Patients were investigated with cervical biopsy, endometrial aspiration or hysteroscopy and curettage. Excluding patients who had hysterectomy and cervical cancer, 97% of the patients had hysteroscopic examination. The majority of patients with postmenopausal bleeding had atrophic vaginitis and/or atrophic endometrium (58.8%). Other common diagnoses were endometrial polyp (9.4%), endometrial carcinoma (9.4%) and carcinoma of the cervix (6%). Other diagnoses included submucous fibroid, endometrial hyperplasia, pyometra, ovarian cancer and urethral caruncle. Pathologies found were similar to early reports, except that incidences of submucous fibroids and endometrial polyps were higher (13.4%). We concluded that all patients with postmenopausal bleeding should be investigated thoroughly as 22% have significant pathology. Hysteroscopy is the preferred investigation because of high incidences of endometrial polyps and submucous fibroids.

Single channel properties of P2X ATP receptors in outside-out patches from rat hippocampal granule cells

The single channel properties of P2X ATP receptors were investigated in outside-out patches from hippocampal granule cells in brain slices from 12-day-old rats. The results demonstrate that functional P2X ATP receptors are expressed in hippocampal granule cells and, combined with previously published information on the P2X subunits expressed in the hippocampus, suggest that the receptors may be heteromers of the P2X4 and P2X6 subunits or P2X1, P2X2, P2X4 and P2X6 subunits. Two distinct types of P2X channel openings were observed. A flickery P2X receptor channel was observed in three patches with a mean chord conductance of 32 +/- 6 pS, a mean open time of 1.0 +/- 0.3 ms and a mean burst length of 11 +/- 5 ms at a membrane potential of -60 mV. A large conductance P2X receptor was observed in 19 out of 98 patches with a mean conductance of 56 +/- 1. 8 pS, a linear current-voltage relationship between -80 and +60 mV with a reversal potential around 0 mV, a mean open time of 2.6 +/- 0. 2 ms and a mean burst length of 8.8 +/- 1.8 ms at -60 mV. At an ATP concentration of 1 mM, these channels exhibited a low steady-state open probability (Popen, 0.07 +/- 0.008; n = 15), little apparent desensitisation and were also activated by alpha,beta-methylene ATP (alpha,beta-meATP, 40 microM; Popen, 0.007 +/- 0.0002; conductance, 57 +/- 1.1 pS; n = 3). No decrease in the single channel conductance was observed on increasing the free extracellular calcium concentration from 0.3 to 0.85 mM. Channel closed time distributions were fitted with five exponential components with time constants (and relative areas) of 90 micros (20 %), 0.77 ms (32 %), 10 ms (15 %), 90 ms (18 %) and 403 ms (15 %) at 1 mM ATP. Of these, the first two components are suggested to represent gaps within single activations of the receptor based on the lack of agonist concentration dependence of these two shut time components between 1 microM and 1 mM ATP. Suramin (40 microM) significantly increased the single channel conductance (19 +/- 7 %; n = 5) and produced a small decrease in Popen (39 +/- 9 %; n = 5) by decreasing mean open time, burst length and total open time per burst. These actions of suramin are not consistent with simple competitive antagonism.

Stepwise pain score analysis of the effect of local lignocaine on outpatient hysteroscopy: a randomized, double-blind, placebo-controlled trial

OBJECTIVE

To assess the efficacy of lignocaine gel in reducing the overall pain and pain of individual steps during outpatient hysteroscopy in comparison with placebo (no anesthesia).

DESIGN

A prospective, randomized, double-blind, placebo-controlled trial.

SETTING

Outpatient hysteroscopy clinic in a regional hospital in Hong Kong.

PATIENT(S)

A total of 500 Chinese patients undergoing outpatient hysteroscopy.

INTERVENTION(S)

Application of lignocaine gel to the cervix during outpatient hysteroscopy.

MAIN OUTCOME MEASURE(S)

Mean pain score using present pain intensity, overall pain score measured by total area under the curve, and the pain score of individual steps in the procedure in patients receiving lignocaine gel were compared with those of patients having no anesthesia. The failure rate and poor-view rate in both groups were also compared.

RESULT(S)

There were no statistically significant differences in mean pain score, overall pain score, and pain score of individual steps between the lignocaine group and controls. The failure rate and poor-view rate also showed no statistically significant differences.

CONCLUSION(S)

Outpatient hysteroscopy without anesthesia is acceptable to most Chinese women, and the local application of lignocaine gel is not effective in reducing pain.

Anaesthetic management of a patient with low tracheal obstruction requiring placement of a T-Y stent

A 60-year-old man with a history of oesophagectomy for carcinoma presented with worsening dysphagia, dyspnoea and tachypnoea secondary to oesophageal stricture and tumour invasion of the trachea causing a "ball-valve" obstruction. The patient required placement of a T-Y tracheobronchial stent to relieve the airway obstruction and was successfully managed using sevoflurane for gaseous induction followed by maintenance with propofol total intravenous anaesthesia.

First laboratory observation of freely propagating electrostatic waves produced by intense, localized field structures

Freely propagating electrostatic waves (free modes) are observed to be produced by intense, localized field structures in laboratory measurements performed in an unmagnetized plasma in a state of strong Langmuir turbulence. The freely traveling waves conform to the Langmuir dispersion relation and have an average frequency which is approximately 11% higher than the average frequency of the localized field structures. The observations are consistent with the predictions of strong Langmuir turbulence theory.

Recent developments in stereotactic breast biopsy methodologies: an update for the surgical pathologist

Recently, there have been a number of new devices introduced for stereotactic biopsy of nonpalpable, mammographically detected lesions. The vacuum-assisted core biopsy (VACB) (Minimal Invasive Breast Biopsy (MIBB), U.S. Surgical, Norwalk, CT; Mammotome, Biopsys Medical, Cincinnati, OH) obtains multiple tissue cores (11-gauge) in a circumferential manner around the biopsy probe, inserted under stereotactic guidance. It provides more complete sampling of mammographic lesions than the conventional 14-gauge stereotactic core biopsy, reducing the number of unsatisfactory biopsies. The advanced breast biopsy instrumentation (ABBI) (United States Surgical Corporation, Norwalk, CT) system utilizes stereotactic technique and an oscillating blade-cutting mechanism to obtain a single large diameter (5 mm to 20 mm) tissue core, with the aim of obtaining an intact lesion in its entirety for histologic assessment. Its potential as a treatment option is still under investigation. Suggested protocols for specimen handling are presented together with a review of the recent literature. Close liaison with radiologists and surgeons performing these biopsies will allow the collection of further outcome data to evaluate the strengths and weaknesses of each technique.

Pigmented central neurocytoma: case report and literature review

A case of pigmented central neurocytoma is reported. The tumor showed histologic, immunophenotypic, and ultrastructural features of central neurocytoma. The pigment consisted of an intimate association of lipofuscin and neuromelanin. Pigmented neuroepithelial tumors are rare, and the pigments may be neuromelanin or melanin. Pigmented central neurocytoma has not been described to date. The low proliferation rate of the tumor suggests a delayed turnover of tumor cells as a possible cause of lipofuscin accumulation. Autocatalytic peroxidation of lipofuscin is a possible mechanism of neuromelanin formation.

Vaginal misoprostol compared with vaginal gemeprost in termination of second trimester pregnancy. A randomized trial

A prospective randomized trial was conducted in 140 women to compare the efficacy of vaginal gemeprost with vaginal misoprostol for termination of second trimester pregnancy. Women requesting termination of second trimester pregnancy were randomized into two groups. Group A women were given 1 mg vaginal gemeprost every 3 h for a maximum of five doses in the first 24 h, whereas group B women were given 400 micrograms vaginal misoprostol every 3 h for a maximum of five doses in 24 h. The median induction-abortion interval in the vaginal misoprostol group (14.1 h) was significantly shorter than that in the gemeprost group (19.5 h). The percentage of women who achieved successful abortion within 24 h in the misoprostol group (80.0%) was significantly higher than that in the gemeprost group (58.6%). There was no significant difference in the incidence of side effects between the two groups except for diarrhea, which was more common in the gemeprost group. The incidence of fever was more common in the misoprostol group. It is concluded that vaginal misoprostol is more effective than gemeprost in termination of second trimester pregnancy.

Outpatient diagnostic hysteroscopy: analysis of 429 cases

OBJECTIVE

To evaluate the diagnostic accuracy of outpatient hysteroscopy.

METHODS

Outpatient CO2 hysteroscopy using video-camera for monitoring was performed on 429 patients from November 1995 to December 1996. The indications included post-menopausal bleeding (32.2%), menorrhagia (25.4%), irregular bleeding (24.7%), intermenstrual spotting (10.2%) and others (7.5%).

RESULTS

Hysteroscopy was successfully performed on 420 (97.9%) patients. Although no anaesthetics were used, only two (0.47%) patients complained of severe pain during the procedure. Twenty-one (4.9%) patients had poor view due to submucosal fibroid or bloody view. There were no operative complications. Normal uterine cavity was found in 36.8% and atrophic endometrium in 34.7% of patients. The pathologies included submucosal fibroid (10.7%), endometrial polyp (5.8%) and fibroid polyp (2.8%). Five (1.2%) out of 429 patients were confirmed histologically to have endometrial carcinoma. No malignant lesions were missed. A total of 372 (86.7%) patients had histological confirmation of hysteroscopic findings. In 57 (13.2%) patients, the histological results did not correlate with the hysteroscopic findings.

CONCLUSIONS

Outpatient hysteroscopy and suction curettage is a safe, cost-effective and acceptable procedure.

Mucinous anal gland carcinoma with perianal Pagetoid spread

We report a case of well differentiated perianal mucinous carcinoma with associated dysplasia of the adjacent anal gland epithelium. Anal gland dysplasia is only rarely demonstrable histologically in cases of adenocarcinoma of suspected anal gland origin. The tumor was not associated with chronic perianal abscess or fistula formation, which had been regarded as important in the pathogenesis of perianal mucinous carcinoma. There was associated, clinically unsuspected Pegetoid intra-epidermal spread of adenocarcinoma in the perianal skin.

Critical care in obstetrical patients: an eight-year review

OBJECTIVE

To review the clinical characteristics and outcomes of critically ill obstetrical patients and to determine the outcome predictability using the Acute Physiology and Chronic Health Evaluation (APACHE) II scoring system.

METHODS

A retrospective data collection of all obstetrical patients (n = 49) admitted for more than 24 hours to the Intensive Care Unit at Kwong Wah Hospital from 1988 to 1995 was conducted. Demographics, obstetric data, preexistent medical problems, diagnosis, days staying in the ICU and ICU related data were recorded for each patient.

RESULTS

Obstetric admissions to ICU during the study period represented 0.12% of all deliveries during this period. There was a predominance of postpartum admissions and obstetric diagnosis responsible for the patients' critical illness. Massive postpartum haemorrhage was the single most common cause of ICU admission, representing 53.0% of all patients. Preeclampsia and eclampsia (14.3%), anaesthesia related complications (14.3%), and medical diseases complicating pregnancy (14.3%) were the other common disease categories for ICU admission. Two cases (4.1%) of surgical disease complicating pregnancy were admitted. The maternal mortality rate was 5.1 deaths per 100,000 total births, or 2 maternal deaths in 39,354 total deliveries in this study period. All deaths were due to nonobstetric causes. The perinatal mortality rate was 10.0% (5 cases) in this study group.

CONCLUSIONS

When applying the Acute Physiology and Chronic Health Evaluation (APACHE) II scoring system in predicting the final outcome in this group of obstetric patients, we found that our obstetric patients requiring intensive care had a better outcome than predicted, as expressed by a low mortality ratio (0.25).

A voltage-dependent noradrenaline-sensitive intracellular calcium store in smooth muscle cells of the guinea-pig aorta

In this study we investigated the effects of K+ depolarization on the contractile responses of guinea-pig aortic smooth muscle to noradrenaline (NA) and caffeine (CAF) in Ca(2+)-free Krebs solution (CafKS). NA (1 microM) or CAF (10 mM) in regular CafKS induced a (termed first) phasic contraction, but responses to a second NA or CAF stimulation were significantly reduced, suggesting a depletion of the common intracellular (NA/CAF sensitive) store previously reported by investigators. After the depletion of this store (i) neither 105 mM KCl CafKS nor 10 mM CAF in this high K+ solution induced contraction; (ii) NA-induced contractions were enhanced by KCl, increasing dose dependently with KCl ranging from 25 to 105 mM; (iii) in 105 mM KCl CafKS repetitive NA stimulations elicited enhanced and nondecreasing responses equal to or greater than the first contraction, suggesting an efficient recycling of intracellular Ca2+. In the presence of 10 microM cyclopiazonic acid NA-induced contraction was enhanced, but the response to a subsequent NA stimulation in the absence of cyclopiazonic acid was greatly reduced, implying an inhibition of the Ca2+ recycling function; (iv) 2.0 mM EGTA (ethylene glycol bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid) attenuated the NA-induced contractions in regular and 105 mM KCl CafKS. On the basis of the above results we propose that in the smooth muscle cells of guinea-pig aorta, there are two intracellular Ca2+ stores: a NA/CAF-sensitive store and a voltage-dependent NA-sensitive store with effective Ca2+ recycling capability.

Red cell flux during the cardiac cycle in the rabbit myocardial microcirculation

OBJECTIVE

(1) To measure regional phasic myocardial red cell flux during a cardiac cycle using a laser Doppler velocimeter. (2) To test the responses of regional red cell flux to a vasodilator (adenosine), a vasoconstrictor (angiotensin II), and an inotrope (isoproterenol).

METHODS

Using an anaesthetised open-chest rabbit with the pericardium intact a 140-micron-tip fibre optic probe was placed in the left ventricular myocardium in various locations. With the fibre in place drugs were given to alter myocardial loading conditions while red cell flux was registered.

RESULTS

Phasic red cell flux was similar in the epicardium to endocardium giving an average endo/epi ratio of 1.14 in the rabbit heart. At least two peaks of increased red cell flux within a single cardiac cycle were observed. Some unique patterns for red cell flux were observed in specialised myocardial structures. Adenosine increased red cell flux but minimally changed the pattern of phasic flux throughout the cycle.

CONCLUSIONS

Laser Doppler velocimetry permits the recording of phasic red cell flux during the cardiac cycle in the myocardial microcirculation. Its pattern is determined by both coronary arterial inflow and venous outflow. The pattern of red cell flux may be characteristic for a region-probably determined by difference in tissue pressure (attributable to the pattern of muscle fibre shortening and collagen tethering) and changes in capillary length and density.

Endothelin-induced electrical activity and calcium dynamics in vascular smooth muscle cells: a model study

A model is proposed to describe the electrical activity and intracellular calcium dynamics of vascular smooth muscle cells (SMC) induced by endothelin (ET1). The conductance of the nonselective channels (NSCs), proportional to the ET1-receptor complex (ET . R), is intracellular calcium dependent. Inositol (1,4,5)-trisphosphate (IP3) produced by ET1 releases Ca2+ from the IP3-sensitive Ca2+ store. The transient increase of intracellular Ca2+ triggers the release of Ca2+ from the Ca(2+)-sensitive store by a Ca(2+)-induced Ca2+ (CICR) mechanism and activates the Ca(2+)-activated K+ current (IK,Ca). The inward current (Iin) via the NSC can depolarize the cell to a level at which the L-type Ca2+ current becomes activated (ICa). The level of depolarization is determined by the relative amplitude of (Iin + ICa + IK,Ca) and the voltage- and time-dependent K+ current. The model simulations show that (a) in cells without a CICR mechanism, short-lasting stimulation by ET1 elicits higher membrane potential and Ca2+ than long-lasting stimulation; (b) in cells with or without a CICR mechanism, a reduction of normal membrane capacitance (1 muf/cm2) results in either significant and sustaining or oscillatory membrane potential and intracellular calcium concentration. The applicability of the model to the study of electrical activity and calcium dynamics associated with hypercholesterolemia is discussed.

A model of electrical activity and cytosolic calcium dynamics in vascular endothelial cells in response to fluid shear stress

A mathematical model is proposed to describe the intracellular Ca2+ (Cai) transient and electrical activity of vascular endothelial cells (VEC) elicited by fluid shear stress (tau). The intracellular Ca2+ store of the model VEC is comprised of a Cai-sensitive (sc) and an inositol (1,4,5)-trisphosphate (IP3)-sensitive compartment (dc). The dc [Ca2+] is refilled by the sc whose [Ca2+] is the same as extracellular [Ca2+]. IP3 produced by the tau-deformed mechanoreceptors discharges the dc Ca2+ into the cytosol. The increase of cytosolic [Ca2+] induces Ca2+ release (CICR) from the sc. The raised Cai activates a Cai-activated K+ current (IK,Ca) and inhibits IP3 production. The cell membrane potential is determined by IK,Ca, voltage-dependent Na+ and K+ currents. Steady tau > 0.1 dyne/cm2 elicits a Cai transient which reaches peak value at 19-54 sec. The peak Cai varies sigmoidally with Log10(tau) with a maximal peak Cai of 150 nM at tau = 4 dynes/cm2. Step increases of tau fail to elicit a Ca2+ response in cells previously stimulated by a lower shear. The Ca2+ response gradually decreases with repetitive tau stimuli. Pulsatile shear elicits two to three times higher Cai and hyperpolarizes the cell more than steady shear of the same magnitude. The simulated Ca2+ responses to tau are quantitatively and qualitatively similar to those observed in cultured VEC. The model provides a possible explanation of why the vasodilating stimulus is greater for pulsatile flow than for nonpulsatile flow.

Comparing threshold visual fields between the Dicon TKS 4000 automated perimeter and the Humphrey Field Analyzer

BACKGROUND

The automated perimeter is becoming the instrument of choice in the analysis of the visual field. There are a number of different perimeters in use and it can be difficult to compare results from different instruments. The purpose of this study was to compare visual field threshold measurements determined by the Humphrey Field Analyzer and the Dicon TKS 4000.

METHODS

This study provided a statistical comparison of field test results from the Dicon TKS 4000 Automated Perimeter (program number 9) and the Humphrey Field Analyzer (30-2 threshold using FastPac). Central 30 degree, 76 point full threshold fields were performed on 20 non-dilated optometry students (age range 21 to 32 years, mean age 25.1 years) using both instruments. For the purpose of statistical analysis, the visual field was divided into 24 sectors, each measuring 10 degrees by 10 degrees. The mean threshold value of each group of points was compared between instruments.

RESULTS

A difference was found between instruments (significant at P&0.0001) for 19 of 24 sectors. In addition, the Humphrey Field Analyzer threshold values were consistently higher (average difference 2.47 dB) than the Dicon values. Each subject was tested twice on each instrument; both instruments showed high test-retest correlations: 0.970 for the Humphrey and 0.971 for the Dicon. Test administration times were comparable (average 435.2 seconds for the Humphrey Field Analyzer and 430.2 seconds for the Dicon), although the Dicon averaged 41.5 fewer presentations tested per examination. Fixation losses were higher with the Dicon instrument.

CONCLUSIONS

A practitioner may be reluctant to directly compare test results between the two instruments since they produce threshold values of statistically significant differences. The Dicon TKS 4000 was subjectively preferred over the Humphrey Field Analyzer by most subjects.

A kinetic model of coronary reactive hyperemic response to transient ischemia

A kinetic model is proposed to delineate the factors that determine the coronary reactive hyperemic response (RHR) to transient ischemia. The model comprises of myocardial-interstitial (M) and vascular (V) compartments. Vasodilator metabolites (VM) are produced in the M compartment during the interval of coronary occlusion. The rate of VM production is dependent on the flow rate during the ischemic period, the ratio of excess flow above the control level (R) to the loss of flow during occlusion period (D), the amount of oxygen stored and the degree of vasodilation in the V compartment prior to occlusion. Following a complete release of occlusion, VM are transported from the M to V compartment and are washed out or degraded with time. The time course of RHR is determined by the coronary patency which is proportional to VM concentration in the V compartment. Based on a set of numerical constants, the model is tested by simulating RHR to the various occlusion manoeuvres: a pair of 10 sec occlusions separated by brief release, a 15 sec release followed by a second brief occlusion, a brief release of an occlusion followed by restricted inflow and a period of restricted inflow after occlusion. The simulated results fit the experimental R/D and RH durations data of canine hearts. Factors that determine the impairment of RH capacity in coronary stenosis are suggested in terms of the model scheme.

A model of calcium regulation in smooth muscle cell

A model of smooth muscle cell is proposed to describe the mechanisms by which intracellular calcium (Cai) is regulated when the cell is stimulated by noradrenaline (NA)/agonist (Ag) or depolarized. The intracellular Ca2+ store is comprised of two calcium pools: A-store which is sensitive to inositol 1,4,5 trisphosphate (IP3) and C-store, insensitive to IP3 but sensitive to Cai. The A-store is refilled by Ca2+ from C-store and reuptake of Cai. The uptake rate is dependent on the degree of filling in the A-store. The C-store, activated by Cai, is replenished by extracellular Ca2+ (Cao). IP3 generated by NA or Ag changes the A-store permeability, discharging its Ca2+ into the cytosol. Depolarization activates Ca2+ current through the L-type channels. The following events are simulated and compared with experimental observations: (a) Cai transient stimulated by high K+; (b) Cai transient and Ca2+ efflux induced by shortlasting and longlasting NA in the absence of Cao; (c) effect of lanthanum on NA-induced Cai and Ca2+ efflux; and (d) effects of nifedipine, ryanodine, BAY K 8644 and cyclopiazonic acid on Cai.

A model of cytosolic calcium regulation and autacoids production in vascular endothelial cell

A model of vascular endothelial cell is proposed to describe the mechanisms by which cytosolic calcium (Cai) is modulated and endothelium-derived relaxing factor (EDRF) and prostacyclin (PGI2) are released when the cell is stimulated by agonist. The intracellular Ca2+ store of the model cell is comprised of a superficial (sc) and a deep (dc) compartment. The dc Ca2+ content is refilled by the sc whose [Ca2+] is the same as extracellular Ca2+. Inositol (1,4,5)-trisphosphate (IP3) produced by agonist modifies the dc permeability which discharges its Ca2+ to the cytosol. The increase of Cai induces Ca2+ released from the sc. Ca(2+)-activated K+ current hyperpolarises the cell. The raised Cai releases PGI2 in the presence of IP3 while EDRF is released by Cai. The model explains satisfactorily the Ca2+ transient and autacoids production of the aortic endothelial cell without the need of calcium influx from extracellular space. The cytoplasmic Ca2+ oscillations observed in human endothelial cell from umbilical veins were reproduced by the model. Production of EDRF by the artery due to increase in pressure was also simulated.

Functional recovery following direct or graft repair of nerve gaps in the rat

The tibial functional index has been employed to assess functional recovery in rats following nerve repair by direct coaptation under tension or by grafting. Recovery after direct repair of small to moderate gap sizes was better than that after grafting.

Vasomotor coronary oscillations: a model to evaluate autoregulation

A simple model was proposed to characterise the oscillatory and nonoscillatory pattern of canine coronary circulation responses induced by a small dose of a vasodilator adenosine or the Ca2+ channel blocker diltiazem. This model consists of two differential equations describing the interaction of dilating (D) and constricting (C) resistance components. With the assumption that the rate constants associated with (D) were dependent on adenosine concentration and those associated with (C) were a function of Ca2+ channels, the model predicted: a) a damped oscillation of resistance to flow at low dose of adenosine, b) a predominant vasodilation at high dose of adenosine, and c) a sustained vasodilation in response to diltiazem. Parameters characterising the coronary resistance were evaluated by fitting the model results to calculated resistance from measured coronary flow and aortic pressure. As well, the model predicted accurately the peak resistance to great cardiac and coronary sinus venous flow in patients. This study indicates that the oscillation frequency of coronary resistance induced by a low dose of adenosine (0.01 mg/kg) is indicative of the uptake rate of adenosine by the heart and the coronary resistance provides considerable information on vasomotor control of the coronary circulation.

Systolic pressure gradients between the wall of the left ventricle, the left ventricular chamber, and the aorta during positive inotropic states: implications for left ventricular efficiency

To study systolic pressure gradients developed between the left ventricular wall, its chamber, and the aortic root, in one group of dogs left ventricle ventral wall intramyocardial pressure, left ventricular outflow tract pressure, and aorta pressure were compared with aortic flow as well as left ventricular dimension changes during control conditions as well as during positive intropic states induced by isoproterenol, stellate ganglion stimulation, and noradrenaline. In another group of dogs systolic pressures in the ventral wall of the left ventricle, the main portion of the left ventricular chamber, and the aorta were compared with aortic flow during similar interventions, before and after the administration of phentolamine. Pressure gradients between the wall of the left ventricle and the outflow tract of the left ventricle were minimal during control states, but during the three positive inotropic states were increased significantly. In contrast, pressure gradients between the outflow tract of the left ventricle and the aortic root were insignificant during positive inotropic states; those between the wall and main portion of the chamber were only significantly different during left stellate ganglion stimulation. The data derived from these experiments indicate that useful peak power output of the left ventricle (systolic aortic pressure X flow) is unchanged following isoproterenol infusion, but is increased by stellate ganglion stimulation and noradrenaline. The useful peak power output index (an index of left ventricular efficiency derived by dividing useful peak power output by peak intramyocardial pressure) was reduced more by isoproterenol than the other two interventions.(ABSTRACT TRUNCATED AT 250 WORDS)

Model of calcium movements in the mammalian myocardium: interval-strength relationship

A model is proposed to describe the interval-strength relationship in mammalian cardiac muscle in terms of "discrete" calcium movements associated with each cycle. The sarcoplasmic reticulum is assumed to be comprised of three functional sub-compartments: (1) The "main calcium store" which contains most of the calcium (predominantly bound) and is considered, due to its large buffering capacity, to account for the "long-term memory" lasting 7-10 beats. (2) The "releasable terminal" which contains the calcium readily available for release (all or most of it free) and accounts for the "short-term memory" which affects the subsequent beat. (3) The longitudinal network of the SR recirculating the myofibrillar calcium to the "main calcium store". The total content of calcium in the main store is determined by the transsarcolemmal influx and efflux. While influx occurs only during depolarization, efflux occurs during the whole cardiac cycle. The amount of free calcium in the main store is determined by an equilibrium equation. The release of calcium from the "releasable terminal" is governed by a "concentration-dependent" mechanism. This implies that when the concentration in the "releasable terminal" increases, the fraction released increases and the residual calcium left for the subsequent contraction decreases. The model predicts the following interval-strength relationships: steady state peak tension; changes from one steady rate to another; restitution curves; post-stimulation potentiation; paired stimulation; premature beats; post-extrasystolic potentiation following interpolated, basal or complimentary interval.

The dynamics of the coronary venous system in the dog

The coronary vascular system is characterised by a small blood-volume, high resistance arterial system, in which flow is primarily diastolic, a capillary-venule exchange system and a large blood volume, low resistance venous system with predominantly systolic flow. In order to understand the venous component we measured intramyocardial pressure (IMP), peripheral (PVP) and central (CVP) coronary venous pressure, as well as phasic coronary sinus outflow. Based on the experimental data, a model is proposed to describe the hemodynamics of the coronary venous system. This model consists of an intramyocardial vascular storage region into which the arterial blood flows during diastole. During systole, the pressure in the storage vessels increases, pushing the peripheral and epicardial venous system. Blocking the inflow tends to empty the 'storage' and the peripheral regions, reducing venous pressures. Occlusion of the outflow increases the blood volume in these regions, elevating venous pressures. The observations of peak (IMP-PVP) vs peak venous flow during vagal, right abd left stellate stimulation conform satisfactorily with the model.

Hemodynamics of coronary artery stenosis

A model is proposed to study the hemodynamics of various types of coronary stenosis. The model coronary artery is assumed to be an elastic tapered tube. A progressive degree of concentric and eccentric stenoses are studied. Measured pulsatile coronary pressure, flow, and intramyocardial pressure are used as input data to calculate the pressure and flow velocities at different locations of the artery. The simulation yields results that agree well with published data in dog experiments, and those from human stenotic coronary arteries. The present model shows that in a cardiac cycle, the overall hydraulic resistance owing to a specific stenosis tends to be flow independent at low flow rate but increases linearly with flow at higher coronary flow. This flow independent resistance increases with progressive stenosis. At low flow, the mean coronary flow in a cardiac cycle is relatively constant with stenoses up to 80%, but decreases dramatically with further increase in the degree of narrowing. At high resting flow rate, this mean flow is markedly reduced at much smaller degrees of constriction. The simulated pressure velocity relation of poststenotic dilatation indicates an additional pressure loss at the distal end of the stenosis, but the calculated resistance to flow is actually lessened. While stenosis length increases pressure loss and resistance to flow, its effect on mean flow appears disproportionally insignificant. Eccentric lesions appear to be more detrimental than concentric ones as they produce additional pressure loss and greater resistance across the coronary artery lesion.

Coronary artery compliance in the dog

Measurement of left anterior descending coronary arterial pressure, phasic coronary flow, and intramyocardial pressure in an open-chest dog provided data, which when entered into the computer model of the coronary circulation, permitted calculation of coronary artery compliance and resistance during systole and diastole. Resting in vivo compliance averaged 0.21 x 10(-3) mL/mmHg (1 mmHg = 133.322 Pa) while systolic resistance averaged 4.05 mmHg X min-1 X mL-1 and during diastole 2.06 mmHg X min-1 X mL-1. Left stellate ganglion stimulation or vasodilation caused minimal changes in compliance but glutaraldehyde applied to arterial wall caused a decrease in compliance. Sympathetic stimulation and vasodilation decreased both diastolic and systolic resistance. Transmural distribution of coronary flow was not significantly altered by the experimental changes in compliance and resistance.