Filgrastim. A reappraisal of pharmacoeconomic considerations in the prophylaxis and treatment of chemotherapy-induced neutropenia.

Neutropenia is a frequent and often dose-limiting complication of chemotherapy and is associated with considerable patient morbidity and mortality. Standard treatment in patients who become febrile includes hospitalisation and empirical antibiotic therapy. Filgrastim is a recombinant human granulocyte colony-stimulating factor (rHuG-CSF). It significantly decreases the incidence of febrile neutropenia in patients receiving standard-dose chemotherapy, and shortens the duration of febrile neutropenia in patients undergoing autologous bone marrow transplantation (BMT) or peripheral blood progenitor cell (PBPC) infusion after myeloablative chemotherapy regimens. These effects are usually associated with a decrease in hospitalisation and antibiotic requirements. The contribution of filgrastim therapy to beneficial effects on other clinically important end-points (e.g. quality of life, tumour relapse rate, and short and long term survival) remains to be accurately determined. Pharmacoeconomic data concerning the use of filgrastim as an adjunct to standard-dose chemotherapy are derived largely from the results of phase III trials. Cost analyses based on hospital charges suggest that the cost of providing filgrastim therapy can be fully recouped if the drug is used as primary prophylaxis in previously untreated patients, for whom the risk of developing febrile neutropenia is at least 40%. Reserving filgrastim for use in patients who have developed febrile neutropenia in a previous chemotherapy cycle may result in further cost savings. However, careful patient selection is required, since potential cost savings will vary depending upon the risk of hospitalisation in the absence of filgrastim treatment. Infusion of filgrastim-mobilised PBPCs is emerging as a preferred strategy in patients receiving myeloablative chemotherapy, and promising results have been obtained from cost analyses. From a pharmacoeconomic viewpoint, future research should be directed towards defining optimum dosage regimens and hence improving the cost-effective use of filgrastim. Data evaluating patient quality of life and treatment preferences would help define the cost utility of filgrastim therapy. In the meantime, available pharmacoeconomic data support the use of filgrastim as an adjunct to chemotherapy in selected clinical situations.

Pentoxifylline (oxpentifylline). A review of its therapeutic efficacy in the management of peripheral vascular and cerebrovascular disorders.

Pentoxifylline (oxpentifylline) has been used widely in the treatment of intermittent claudication, a prevalent condition in the elderly population. The exact mechanism(s) of action of the drug are unclear, but may be related to identified effects on white blood cell function and haemorrheological parameters. Clinical trials which conform best with European and North American guidelines have shown that 6 months' oral therapy with pentoxifylline 1200 mg/day significantly improves walking distances in patients with intermittent claudication. Patients most likely to benefit from treatment are those with an ankle/arm blood pressure ratio < or = 0.8 and a history of disease > 1 year. However, it remains unclear whether pentoxifylline or any other conservative treatment approach (including physical training) offers long term benefit, as studies comparing the development of intermittent claudication after several years of treatment with the natural course of the disease are still lacking. In patients with more severe vascular disease, intravenous administration of pentoxifylline (1200 mg/day for 21 days) decreased rest pain in patients with critical limb ischaemia. Oral administration (1200 g/day for up to 6 months) increased the healing of venous ulcers of the leg when used as an adjunct to standard compression bandaging. However, further studies are required to confirm these initial findings. The efficacy of pentoxifylline in the treatment of cerebrovascular disease has been evaluated in controlled clinical trials. Most notably, long term therapy (1200 mg/day) may slow the progression of dementia in patients who meet the clinical diagnostic criteria for 'multi-infarct' dementia and who also have clinical and neuroradiological evidence of cerebrovascular disease. The drug is effective in decreasing the risk of transient ischaemic attacks, but there are insufficient data to determine its value in the prevention and treatment of stroke. Pentoxifylline is well tolerated, with gastrointestinal effects reported in fewer than 3% of treated patients. However, the incidence of adverse events may be higher in elderly patients and/or those receiving concomitant medications. In summary, pentoxifylline is the most established agent when drug therapy is deemed appropriate in patients with intermittent claudication. Moreover, a promising new development for the drug is in the management of cerebrovascular dementia, an area where few therapeutic options are currently available.

Lenograstim. A review of its pharmacological properties and therapeutic efficacy in neutropenia and related clinical settings.

Lenograstim is a recombinant glycosylated human granulocyte colony-stimulating factor (rHuG-CSF) which principally regulates the formation and function of neutrophils. Like other colony-stimulating factors (CSFs), lenograstim has been developed for the prevention and treatment of iatrogenic and disease-related neutropenic conditions. In phase III clinical studies, prophylactic administration of lenograstim shortened the duration of chemotherapy-induced neutropenia in patients with nonmyelogenous cancers who received standard-dose chemotherapy or myeloablative regimens followed by bone marrow transplantation (BMT). A decrease in the incidence of infection after standard regimens and fewer days with infectious and febrile neutropenic episodes during recovery from BMT occurred concomitantly with the amelioration of neutropenia. In each setting, the decrease in morbidity was associated with shorter hospitalisation times and reduced administration of parenteral antibacterial agents. As with another rHuG-CSF, filgrastim, bone pain (non-serious) was the most common adverse reaction to lenograstim therapy. This occurred in 13% of lenograstim recipients and 5% of placebo recipients treated for chemotherapy-induced neutropenia with standard regimens. Lenograstim may facilitate dose optimisation and permit limited dose intensification of standard chemotherapy. Furthermore, the drug, used alone or in combination with chemotherapy, is effective in mobilising peripheral blood progenitor cells (PBPCs) for subsequent reinfusion. The latter is a promising technique which may supplement or ultimately replace BMT for stem cell rescue after myeloablative chemotherapy. However, it has yet to be established whether the dose intensification achievable with lenograstim and/or stem cell rescue has a material effect on relapse-free and survival times. Preliminary data suggest that lenograstim is effective in increasing the neutrophil count in patients with severe chronic neutropenia (Kostmann's syndrome), as well as patients with AIDS or AIDS-related complex with zidovudine-induced neutropenia. Thus, lenograstim, like other CSFs, is a valuable adjunct to cytotoxic chemotherapy for the treatment of nonmyelogenous cancers, including myeloablative regimens followed by stem cell rescue with BMT and/or PBPC infusion. Future clinical experience is likely to confirm the usefulness of the drug in the management of disease-related neutropenia, myeloid disorders and neutropenia in patients with AIDS.

Ramipril. An updated review of its therapeutic use in essential hypertension and heart failure.

Ramipril is a second generation angiotensin converting enzyme (ACE) inhibitor. Like enalapril, it is a prodrug and is hydrolysed in vivo to release the active metabolite, ramiprilat, which has a long elimination half-life, permitting once-daily administration. The antihypertensive efficacy of ramipril has been confirmed in large-scale noncomparative studies conducted in general practice as well as in more rigorously controlled clinical trials. In the former, approximately 85% of patients with mild to moderate essential hypertension have responded successfully to treatment with ramipril 2.5 or 5 mg/day, while comparative trials indicate that the antihypertensive efficacy of the drug is equivalent to that of other established ACE inhibitors and the beta-adrenoceptor antagonist atenolol. As expected, the response rate to ramipril monotherapy is lower in patients with severe hypertension (around 40%), although the blood pressure lowering effect can be enhanced with the addition of a diuretic such as hydrochlorothiazide or piretanide. The antihypertensive efficacy of ramipril is maintained in patients with diabetes mellitus and preliminary data indicate that the drug has the beneficial effect of decreasing urinary albumin excretion in diabetic patients with nephropathy. Ramipril is superior to atenolol in causing regression of left ventricular hypertrophy, although the clinical significance of this effect per se remains to be established. The large-scale Acute Infarction Ramipril Efficacy (AIRE) study demonstrated that ramipril 5 or 10 mg/day significantly decreased the risk of all-cause mortality by 27% in patients with clinical evidence of heart failure after acute myocardial infarction, even if transient. The beneficial effect of ramipril was apparent by 30 days of treatment and appeared to be greatest in patients with more severe ventricular damage after infarction. Ramipril is well tolerated in general practice, with 5% or fewer patients discontinuing therapy because of drug intolerance. The data available suggest that ramipril shares a similar tolerability profile to that of other established ACE inhibitors. Thus, clinical data confirm ramipril as a useful alternative ACE inhibitor for the treatment of patients with mild to moderate hypertension, and indicate a beneficial effect of the drug in patients with clinical evidence of heart failure after acute myocardial infarction. It is also reasonable to assume that ramipril will be of value in the treatment of patients with more established heart failure or asymptomatic left ventricular dysfunction.

Filgrastim. A review of its pharmacological properties and therapeutic efficacy in neutropenia.

Filgrastim, a recombinant human granulocyte colony-stimulating factor (G-CSF), has identical biological activity to that of endogenous human G-CSF, but differs in that it contains an N-terminal methionine residue and is not glycosylated. It principally stimulates activation, proliferation and differentiation of neutrophil progenitor cells and has been evaluated in the treatment of patients with various neutropenic conditions, both iatrogenic and disease-related. Two comparative studies have demonstrated that prophylactic administration of filgrastim 230 micrograms/m2/day significantly reduces the incidence, duration and severity of neutropenia in patients with previously untreated small-cell lung cancer receiving standard-dose chemotherapy with CDE (cyclophosphamide, doxorubicin plus etoposide). Concomitant with the amelioration of neutropenia, the incidence of febrile neutropenia was significantly reduced by 50% and there were 35 and 50% decreases in hospitalisation rates and intravenous antibiotic requirements. Since not all patients receiving standard-dose chemotherapy are at risk of infectious complications, prophylactic filgrastim use may be reserved for those patients who have developed febrile neutropenia during a previous cycle of the same regimen. This strategy may prove less costly, although potential savings must be weighed against a greater risk of patient morbidity and reduced quality of life. When combined with standard intravenous antibiotic therapy, filgrastim further decreases morbidity in patients with established febrile neutropenia and may have a positive impact on overall treatment costs by shortening the length of hospitalisation. Attention is focused on the use of haematopoietic growth factors to support dose-intensification of chemotherapy with a view to improving treatment outcomes in patients with chemo-responsive tumours. Filgrastim, used alone, permits modest increases in dose-intensity and/or dose-escalation of some standard-dose chemotherapy regimens. Moreover, the drug has proven useful as an adjunct to myeloablative chemotherapy followed by stem cell rescue with autologous bone marrow transplantation and/or peripheral blood progenitor cells. However, the impact of these dose-intensification approaches on survival remains to be determined in well-controlled clinical studies. Filgrastim is effective in increasing the neutrophil count and decreasing morbidity in patients with severe chronic neutropenia, including Kostmann's syndrome, and in idiopathic and cyclic neutropenia. In addition, filgrastim has accelerated neutrophil recovery in patients with idiosyncratic drug-induced agranulocytosis. Available data indicate that filgrastim is generally well tolerated. The most frequent adverse reaction is mild to moderate medullary bone pain, reported by approximately 20% of patients, although this can generally be controlled using simple analgesics without the need to discontinue treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Ranitidine: a pharmacoeconomic evaluation of its use in acid-related disorders.

Ranitidine is a histamine H 2-receptor antagonist which, on the basis of its well established tolerability and efficacy profile, has been widely prescribed for the treatment of ulcer disease and mild to moderate reflux oesophagitis. However, the advent of more powerful gastric acid inhibitors (e.g. acid pump inhibitors) and the realisation of the role of Helicobactor pylori infection in duodenal ulcer disease could have considerable clinical and economic implications for the use of ranitidine (and other H 2-receptor antagonists). Simulation modelling studies based on current pricing policies in Europe predict that ranitidine-based treatment will be less cost effective than omeprazole in the short term healing of duodenal ulcer and reflux oesophagitis disease. During longer term treatment, omeprazole is expected to be the dominating strategy over ranitidine-based therapy in Europe and the US. However, the inherent limitations of modelling studies reinforce the need for randomised prospective trials, preferably conducted in a general practice setting and including a quality-of-life analysis. Of the currently accepted approaches for the long term management of recurrent duodenal ulcer disease, daily maintenance therapy with ranitidine has been shown to be more cost effective than intermittent treatment for up to 2 years in the US. The annual cost of providing continuous maintenance therapy with ranitidine 150 mg/day is higher than with cimetidine 400 mg/day, although the extra benefits include a reduced risk of pain and discomfort from an expected lower rate of ulcer recurrence with ranitidine. Simultaneous ulcer healing and eradication of H. pylori markedly reduces relapse rates and is likely to become the management strategy of choice in H. pylori-positive patients, particularly with the advent of more convenient, well tolerated and effective regimens. Moreover, widespread clinical acceptance of H. pylori eradication may yield substantial cost savings to society by reducing the overall need for long term antisecretory therapy. Nonetheless, maintenance therapy with histamine H 2-receptor antagonists remains indicated for patients at high risk of ulcer recurrence who are poorly responsive to or cannot tolerate H. pylori eradication regimens. In summary, the proven efficacy and tolerability of ranitidine will ensure its continued use in the treatment of many patients with duodenal ulcer and mild to moderate reflux oesophagitis. However, there is increasing clinical and economic data favouring the selection of omeprazole in patients with more severe symptoms of these diseases.(ABSTRACT TRUNCATED AT 400 WORDS)

Parnaparin. A review of its pharmacology, and clinical application in the prevention and treatment of thromboembolic and other vascular disorders.

Parnaparin is a low molecular weight (LMW) heparin which, like other members of its class, apparently demonstrates a greater antithrombotic effect relative to its anticoagulant activity when compared with the unfractionated heparin (heparin) from which it is derived. Moreover, subcutaneous parnaparin has a greater bioavailability and longer half-life than heparin, permitting once-daily administration for the prophylaxis of deep venous thrombosis (DVT) or the treatment of established vascular disorders. Prophylaxis with a 7-day regimen of parnaparin 3200 or 6400 IUaXa/day has consistently been associated with a lower incidence of confirmed DVT compared with usual prophylactic regimens of heparin. This intertreatment difference reached statistical significance in a large multicentre study involving a total of 610 surgical patients (3.2% for parnaparin vs 6.3% for heparin). Thus far, however, comparisons of parnaparin with other LMW heparins for this indication are unavailable. Parnaparin has demonstrated equivalent efficacy to heparin in the treatment of established vascular disorders, including phlebopathies and related syndromes, as well as peripheral arterial occlusive disease. Parnaparin also showed some benefit as an adjunctive therapy in patients with angina pectoris. The risk of general bleeding appears to be similar with parnaparin or heparin, although parnaparin results in fewer haematomas at the site of injection, partly because of the less frequent administration regimen. Parnaparin has also been associated with a lower incidence of pain and/or burning sensation at the injection site compared with heparin. As with other LMW heparins, the possibility that parnaparin will be infrequently associated with thrombocytopenia cannot be excluded. Thus, parnaparin may be preferred over traditional heparin for the prophylaxis of thromboembolic events in surgical patients (particularly those at high risk for DVT), as well as the treatment of established vascular disorders with a thrombotic aetiology. Compared with heparin, parnaparin offers the advantages of a more convenient administration regimen coupled with improved local tolerability. However, the therapeutic advantages of parnaparin relative to other LMW heparins have yet to be established in large scale comparative trials.

Cyclosporin: a pharmacoeconomic evaluation of its use in renal transplantation.

Cyclosporin is a powerful immunosuppressive agent which has markedly improved the outcome of renal transplantation, a technique now well established as the treatment of choice for patients with end-stage renal disease (ESRD). Comparison of cyclosporin-based regimens with a regimen of azathioprine and a glucocorticoid (conventional immunosuppression) indicates an improved clinical course and increased long term graft survival rate with cyclosporin which is most apparent in the recipients of a kidney from a living related donor (human leucocyte antigen mismatched) or cadaver donor source. Cyclosporin has also improved the clinical outcome in patient subgroups previously associated with a higher risk of graft failure with conventional immunosuppression, namely older patients and those with diabetes mellitus. Cost-of-treatment studies conducted over the first post-transplant year in the US have found that higher pharmacy charges with cyclosporin-based regimens are associated with lower hospitalisation charges and total billed charges compared with conventional immunosuppression. Lower hospitalisation charges reflect an improved post-transplant clinical course with cyclosporin and this has been associated with an improved quality of life in the recipients of a cadaver donor kidney. Longer term, the direct cost to society of using cyclosporin-based dual therapy (in combination with a glucocorticoid) may be higher than that with conventional immunosuppression, although the difference is likely to be small compared with the ongoing cost of dialysis. Clinical research continues to focus on modified regimens which maximise the clinical benefits of cyclosporin while minimising the associated adverse events, in particular the potential for nephrotoxicity. Sequential drug therapy (induction with globulin, azathioprine and a glucocorticoid followed by delayed administration of cyclosporin) has been associated with an improved clinical course compared with dual therapy, as well as cost containment to a level comparable to that for conventional immunosuppression. Compared with sequential therapy, triple drug therapy (cyclosporin, azathioprine plus a glucocorticoid) has been associated with a similar clinical course and lower acquisition cost during the first post-transplant year, although its overall impact on the longer term cost of transplantation has yet to be assessed. Elimination of cyclosporin from the immunosuppressive protocol of carefully selected patients can be safely achieved during the first post-transplant year. However, it remains to be established whether savings in the long term acquisition cost of immunosuppression are more than offset by the cost of treating a potentially less favourable clinical course.(ABSTRACT TRUNCATED AT 400 WORDS)

Nicardipine. A review of its pharmacology and therapeutic efficacy in older patients.

Nicardipine is a second generation dihydropyridine calcium antagonist which selectively inhibits vascular smooth muscle contraction. In elderly patients, the drug has demonstrated clinical efficacy in the management of hypertension, angina pectoris and ischaemia-related cerebrovascular disease. In particular, nicardipine effectively controls blood pressure in elderly hypertensive patients with or without coexistent disease. In noncomparative trials, a regimen containing nicardipine has been associated with an improvement of symptoms in hypertensive patients with concurrent coronary artery, cerebrovascular or peripheral vascular disease, while in essentially 'healthy' elderly hypertensive patients, nicardipine monotherapy has resulted in improved indices of mobility and cognitive function. As yet, however, there is no evidence that nicardipine (and/or other calcium channel antagonists) decreases cardiovascular morbidity and mortality in elderly patients, as has been demonstrated for more established antihypertensive therapies, namely diuretics and/or beta-blockers. The pharmacokinetic properties of nicardipine in elderly hypertensive patients appear to be similar to those in younger patients. The main adverse events associated with nicardipine in the elderly are related to the vasodilator properties of the drug and include pedal oedema, headache and flushing. However, the drug does not exacerbate spontaneous postural hypotension in the elderly, nor does it adversely affect the coronary artery disease risk profile, even in patients with type II diabetes mellitus. In summary, widespread clinical experience in the elderly indicates that nicardipine monotherapy or a regimen containing nicardipine is useful for the treatment of hypertension, particularly in patients with coexistent coronary artery, cerebrovascular or peripheral vascular disease. Nicardipine monotherapy has also demonstrated efficacy in angina pectoris and shown promise in the management of ischaemia-related cerebrovascular diseases, notably subarachnoid haemorrhage.

Cefpodoxime proxetil. A review of its antibacterial activity, pharmacokinetic properties and therapeutic potential.

Cefpodoxime proxetil is an orally administered prodrug which is absorbed and de-esterified by the intestinal mucosa to release the third generation cephalosporin, cefpodoxime. Cefpodoxime is stable towards the most commonly found plasmid-mediated beta-lactamases and the drug has a broad spectrum of antibacterial activity encompassing both Gram-negative and Gram-positive bacteria, rendering it a possible option for empirical use in a wide range of community acquired infections in both adult and paediatric patients. The extended plasma half-life of cefpodoxime (1.9 to 3.7 h) permits twice daily administration. In comparative trials, twice daily cefpodoxime proxetil (dose equivalent cefpodoxime 100 to 400 mg) was as effective as a 3- to 4-times daily regimen of phenoxymethylpenicillin in pharyngotonsillitis, as well as thrice daily amoxicillin (with or without clavulanic acid) or cefaclor against infections of the ear, the upper and lower respiratory tract, the urinary tract and those of the skin and soft tissues. The latter reflects the enhanced antistaphylococcal activity of cefpodoxime, which distinguishes it from other orally active third generation cephalosporins such as cefixime. Most notably, an oral regimen of cefpodoxime proxetil was as efficacious as parenterally administered ceftriaxone for the treatment of bronchopneumonia in hospitalised patients at risk due to the presence of underlying diseases, addictions or advancing age. A single oral dose of cefpodoxime was also as efficacious as ceftriaxone in uncomplicated anogenital gonococcal infections. Cefpodoxime proxetil is generally well tolerated, with mild to moderate gastrointestinal disturbances occurring in 4 to 15% of patients treated with therapeutic doses. Thus, a convenient twice daily oral regimen of cefpodoxime proxetil can be prescribed as an effective alternative to established beta-lactam therapies in the empirical outpatient treatment of infections of the respiratory and urinary tracts as well as those of the skin and soft tissues.

Contraction and intracellular Ca2+, Na+, and H+ during acidosis in rat ventricular myocytes.

We have investigated the effect of a CO2-induced (respiratory) acidosis on contraction and on intracellular Ca2+, Na+, and pH (measured using the fluorescent dyes fura-2, sodium-binding benzofuran isophthalate, and 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein, respectively) in ventricular myocytes isolated from rat hearts. Initial exposure to acidosis led to a rapid decrease in intracellular pH that was accompanied by an abrupt decline in contractility. There were no consistent changes of intracellular Na+ or Ca2+ during this period. The rapid decline of contractility was followed by a slower partial recovery, which was accompanied by increases in intracellular Na+, systolic and diastolic Ca2+, and an increase in the Ca2+ content of the sarcoplasmic reticulum (estimated using caffeine). Intracellular pH did not change during this slow recovery. The slow rise of intracellular Na+ and the recovery of the twitch were blocked by the Na(+)-H+ exchange inhibitor amiloride. The sarcoplasmic reticulum inhibitor ryanodine blocked the recovery of the twitch but had no effect on the rise of intracellular Na+ induced during acidosis. It is concluded that a major cause of the initial decline of the twitch during acidosis is a decrease in the response of the contractile proteins to Ca2+ due to the decrease of intracellular pH. The subsequent slow recovery of the twitch is due to the decrease of intracellular pH activating the Na(+)-H+ exchange mechanism. This elevates intracellular Na+ and presumably, via the Na(+)-Ca2+ exchange mechanism, intracellular Ca2+. This in turn may lead to increased Ca2+ loading of, and hence release from, the sarcoplasmic reticulum, and it is this that underlies the partial recovery of contraction during acidosis in this preparation.

[Ca2+] and [Na+] in rat ventricular myocytes showing negative and positive force frequency relationships.

Ventricular cells that show a positive force-frequency relationship show graded changes of intracellular [Ca2+] (Cai), [Na+] (Nai) and sarcoplasmic reticulum Ca2+ content with changes of stimulation frequency. Cells that show a negative force-frequency relationship show smaller changes of Nai and no change in the Ca2+ load of the sarcoplasmic reticulum as stimulation frequency is changed.

The effect of a calmodulin inhibitor on intracellular [Ca2+] and contraction in isolated rat ventricular myocytes.

1. The effect of the calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide (W7; 10 microM) on intracellular [Ca2+] ([Ca2+]i) and [H+], and on contraction, has been studied in myocytes isolated from the ventricles of rat hearts. [Ca2+]i and [H+] were monitored using the fluorescent dyes Fura-2 and 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) respectively. 2. W7 decreased the size of both the Fura-2 fluorescence (a function of [Ca2+]i) transient and twitch, but had no effect on their time course. 3. The decrease in the size of the Fura-2 fluorescence transient in the presence of W7 was accompanied by a decrease in the increase of Fura-2 fluorescence that could be elicited by releasing Ca2+ from the sarcoplasmic reticulum using 10 mM-caffeine. 4. There was a decrease in the apparent sensitivity of the contractile proteins to Ca2+ in the presence of W7 which may account, in part, for the decrease in the twitch observed in the presence of W7. 5. Test beats were interpolated at different test intervals after a train of steady-state contractions. Mechanical restitution curves were constructed by plotting the size of the test beat against the test interval. Both the size and the duration of the twitch increased as the test interval was prolonged. W7 slowed this mechanical restitution but had no effect on the changes in the duration of the twitch. 6. Intracellular pH was not altered by W7. 7. These results are discussed in terms of the known actions of calmodulin and W7.

Ca2+ and Na+ in rat myocytes showing different force-frequency relationships.

Intracellular [Ca2+] ([Ca2+]i), intracellular Na+ activity (aiNa), and contraction have been monitored in single myocytes isolated from the ventricles of rat hearts. Some of these cells showed an increase in the size of the twitch as stimulation frequency was increased (positive force-frequency relationship), while others showed a decrease in the strength of contraction as the frequency of stimulation was increased (negative force-frequency relationship). In cells that showed a positive force-frequency relationship, increasing stimulation frequency resulted in increases in aiNa, diastolic [Ca2+]i, systolic [Ca2+]i, and the amount of Ca2+ that could be released from the sarcoplasmic reticulum by caffeine. The rate of decline of the [Ca2+]i transient and the twitch also increased as stimulation frequency was increased. In cells that showed a negative force-frequency relationship, increasing stimulation frequency had less effect on aiNa and had either no effect or decreased systolic [Ca2+]i with no change in the amount of Ca2+ that could be released from the sarcoplasmic reticulum using caffeine. The rate of relaxation of the [Ca2+]i transient and the twitch again increased as stimulation frequency increased. The pattern and time course of mechanical restitution was the same in both cell types. Although these data are essentially descriptive, it is consistent with the hypothesis that the final contractile response observed during changes of stimulation frequency may be dependent on how the Ca2+ loading of the preparation varies with stimulation frequency.

Diastolic, systolic and sarcoplasmic reticulum [Ca2+] during inotropic interventions in isolated rat myocytes.

1. The fluorescent indicator Fura-2 has been used to monitor intracellular [Ca2+] (Ca2+i) in myocytes isolated from the ventricles of rat hearts. 2. The relationships between diastolic Ca2+i, systolic Ca2+i and the Ca2+ content of the sarcoplasmic reticulum (SR; assayed using caffeine) have been studied during changes of stimulation rate and bathing [Ca2+] (Ca2+o). 3. When stimulation rate was increased, there were increases in diastolic Ca2+i, systolic Ca2+i and the Ca2+ content of the SR. 4. The SR inhibitor ryanodine (1 mumol l-1) decreased the size of the Ca2+i transient, and abolished the increase of Ca2+i produced by caffeine (10 mmol l-1). In the presence of ryanodine, increasing stimulation rate increased diastolic Ca2+i but not systolic Ca2+i. 5. Increasing Ca2+o led to increases of diastolic Ca2+i, systolic Ca2+i and SR Ca2+ content similar to those observed during changes in stimulation rate. 6. Ryanodine altered the relationship between systolic and diastolic Ca2+i during changes of Ca2+o. 7. These results are consistent with a change of diastolic Ca2+i leading to an increase in the Ca2+ content of the SR, and hence an increase in the size of the Ca2+i transient during changes in stimulation rate and Ca2+o.

The length, width and volume of isolated rat and ferret ventricular myocytes during twitch contractions and changes in osmotic strength.

The length and width of rat and ferret ventricular myocytes have been measured using a linear photodiode array; the volume of the myocytes has been calculated based on the assumption that the cells were elliptical cylinders. During a twitch contraction, there was a decrease in cell length, but no significant change in the calculated cell volume, because the cells increased in width. Inotropic interventions not only resulted in a greater shortening of the cell during each contraction, but also a greater increase in cell width. Changes in cell length, width and volume on changing the osmotic strength of the bathing solution have also been investigated. The increase in volume in hypotonic solution, and the decrease in hypertonic solution, were the result of changes in the cell width; there were no significant changes in the cell length. It is concluded from the latter experiment that the lateral compliance of a cell is greater than its longitudinal compliance, and, therefore, during a twitch contraction, when the cell shortens, the displacement of the cell contents from the two ends of the cell and the expansion of the cell laterally will not act as a large force to oppose shortening.