Limit cycle oscillations in pacemaker cells.

In recent decades, several mathematical models describing the pacemaker activity of the rabbit sinoatrial node have been developed. We observe that membrane voltage and membrane charge are treated as independent dynamic variables in some of those models, resulting in an infinite number of limit cycles. Then we display numerical results from a new model where membrane voltage (v) is not a dynamic variable, and observe a limit cycle oscillation that can be reached from many different initial conditions.

A theory for the membrane potential of living cells.

We give an explicit formula for the membrane potential of cells in terms of the intracellular and extracellular ionic concentrations, and derive equations for the ionic currents that flow through channels, exchangers and electrogenic pumps. We demonstrate that the work done by the pumps equals the change in potential energy of the cell, plus the energy lost in downhill ionic fluxes through the channels and exchangers. The theory is illustrated in a simple model of spontaneously active cells in the cardiac pacemaker. The model predicts the experimentally observed intracellular ionic concentration of potassium, calcium and sodium. Likewise, the shapes of the simulated action potential and five membrane currents are in good agreement with experiment. We do not see any drift in the values of the concentrations in a long time simulation, and we obtain the same asymptotic values when starting from the full equilibrium situation with equal intracellular and extracellular ionic concentrations.

A possible resolution of the gating paradox.

We introduce a Markov model for the gating of membrane channels. The model features a possible solution to the so-called gating current paradox, namely that the bell-shaped curve that describes the voltage dependence of the kinetics is broader than expected from, and shifted relative to, the sigmoidal curve that describes the voltage dependence of the activation. The model also predicts some temperature dependence of this shift, but presence of the latter has not been tested experimentally so far.

Chaos in weakly-coupled pacemaker cells.

A model of the rabbit sinoatrial action potential is introduced, based on a model by Morris & Lecar. One cell is described by two nonlinear first-order ordinary differential equations, with ten constant parameters. The model is much simpler than most other models in use, but can reproduce perfectly experimentally recorded action potentials. The dynamics of two coupled cells, with and without the presence of periodic acetylcholine pulses, shows examples of bifurcations and strange attractors, mathematical phenomena characterizing chaotic motion. It remains to be clarified whether such dynamics is actually observed, for example in the small irregular variations of the normal heart rate.

Kinetics of mercaptopurine and thioguanine nucleotides in renal transplant recipients during azathioprine treatment.

The purpose of this study was to examine the pharmacokinetics of mercaptopurine (6-MP) and thioguanine nucleotides (6-TGN) during azathioprine treatment. Plasma profiles and urinary excretion of 6-MP and 6-TGN concentrations in red blood cells (RBCs) were measured repeatedly during the first 3 weeks following transplantation in 10 adults, who had received kidney grafts from living related donors. Mean maximal 6-MP plasma concentration (Cmax) was 340 nmol/L (SD = 290), mean time to Cmax (Tmax) was 2 h (SD = 1.8), and mean area under the plasma concentration-time curve (AUC) was 930 nmol/L/h (SD = 770). The mean fraction of azathioprine dose excreted as 6-MP in urine was 1.32% (SD = 1.11). Up to eightfold variability of Cmax and AUC was observed from day to day within each patient. The correlation between 6-MP AUC and amount excreted in the urine was weak (r = 0.37, 95% CI from 0.02 to 0.64). In this group of patients the observed 6-TGN levels in RBCs were low; maxima during the observation period ranged from undetectable to 250 pmol/8 x 10(8) RBCs. In individual patients, 6-TGN levels were relatively stable throughout the dosing interval ("within-dose-interval-CV" < 19%), even when sharp and high 6-MP peaks in plasma were observed.

Environmental hazards in the workplace: legal and safety considerations.

Both the employer and the employee must work together to provide the best possible work environment. The occupational health nurse can facilitate the interaction and communication between these groups to protect the employer and the employee. The company must follow legal guidelines related to hazardous materials, and the employees must understand and adhere to the established policies and procedures developed for their safety. The nurse can help assure that each is protected.

Lack of effect of the calcium antagonist isradipine on cyclosporine pharmacokinetics in renal transplant patients.

Hypertension has emerged as a frequent side effect in transplant recipients on effective doses of cyclosporine (CsA). To control hypertension in renal transplant patients, calcium channel blockers have been used; some of these, however, have been shown to cause significant increases in CsA levels. These findings point out that possible interactions of each calcium antagonist with CsA deserve investigation. We performed an open, placebo-controlled study in 12 stable renal transplant recipients to determine whether short-term isradipine influences CsA pharmacokinetics. All patients had mild to moderate hypertension and received triple immunosuppressive therapy with CsA, azathioprine, and prednisolone. Throughout a 4-week period of isradipine treatment, blood CsA levels (specific and nonspecific monoclonal antibodies) remained stable. The mean trough specific level was 121 +/- 14 micrograms/L following placebo, compared to 120 +/- 14 micrograms/L during isradipine. Corresponding non-specific values were 465 +/- 68 and 474 +/- 63 micrograms/L. Also, values for Cmax, AUC, and t1/2 were not significantly changed following 4 weeks of isradipine. Mean arterial pressure was significantly reduced at the end of the study. This study implies that isradipine does not influence CsA metabolism. Further studies should be carried out to determine its long-term effects on CsA pharmacokinetics and renal function in transplanted patients.

Changes in pharmacokinetics of cisplatin after hepatic resection in rats.

Adjuvant chemotherapy concomitant with hepatic resection has been considered as treatment of patients with hepatocarcinoma. To examine the influence of a partial hepatectomy on the platinum pharmacokinetics, rats were given a single dose of cisplatin (2 mg/kg) prior to 2/3 hepatic resection or sham operation. The partially hepatectomized rats showed a 44% increase in the AUC between 2-48 hr compared with the sham-operated controls. The terminal plasma half-life from 12-48 hr was 21.1 hr for the partially hepatectomized rats and 14.7 hr for the sham-operated animals. Total platinum concentration in liver tissue was 1.3 times that of plasma 20 min after injection. Although cisplatin is considered to be eliminated mainly via renal excretion, our finding may suggest a role for the liver in the elimination of platinum.

Induction of in vitro resistance to 4'-epidoxorubicin and cis-dichlorodiammineplatinum in hepatoma cells.

We have developed in vitro resistance to 4'-epidoxorubicin (Epi-A) and cis-dichlorodiammineplatinum (cis-DDP) in one rat (MH1C1) and one human hepatoma cell line (HepG2). When compared to their parental cells, the Epi-A resistant rat cells were 17 times and the resistant human cells 27 times more resistant to Epi-A in terms of GI50 in the cell growth inhibition assay. The cis-DDP resistant rat cells were 20 times and the resistant human cells 12 times more resistant to cis-DDP. Cross-resistance to cis-DDP was observed in the Epi-A resistant rat cells but not in the human cells. The multidrug resistant gene product, GP 170, was markedly expressed in both Epi-A resistant substrains compared with their parent lines, suggesting a role of this protein in the development of resistance to Epi-A. Cadmium-binding proteins of metallothionein (MT) size bound 52% of cytosolic 109cadmium in the cis-DDP resistant human cells compared with 8% in the parental cells. This may indicate that these proteins contribute to the observed cis-DDP resistance.

The peptide pyroGlu-Gln-Gly-Ser-Asn, isolated from mouse liver, inhibits growth of rat hepatoma cells in vitro.

The peptide pyroGlu-Gln-Gly-Ser-Asn, recently isolated from mouse liver, inhibited DNA synthesis and proliferation in vitro of MH1C1 cells, a rat clonal strain derived from a Morris transplantable hepatoma. Both the biological peptide isolated from mouse liver and the synthetic homolog showed bell-shaped dose-response curves. Maximal inhibition (approximately 50%) was observed at two separate dose ranges: one at 10(-7)-10(-10) M, and one at 10(-14)-10(-15) M.

Contrasting effects of lisinopril and nifedipine on albuminuria and tubular transport functions in insulin dependent diabetics with nephropathy.

An open, randomized, cross-over study was undertaken to assess the effects of lisinopril and nifedipine on albumin excretion, renal haemodynamics and segmental tubular reabsorption in overt diabetic nephropathy. The study consisted of a 4-week run-in period, a 3-week active treatment period, a 4-week wash-out period and a second 3-week active treatment period. Twelve patients with type 1 diabetes with albuminuria, mild to moderate hypertension and a serum creatinine level of less than 200 mumol l-1 were included. Lisinopril reduced albumin excretion from 1343 +/- 337 micrograms min-1 to 879 +/- 299 micrograms min-1 (P less than 0.01), whereas nifedipine was without effect, 1436 +/- 336 micrograms min-1 vs. 1319 +/- 342 micrograms min-1. Glomerular filtration rate (GFR) was unchanged by either drug. Both drugs increased effective renal plasma flow (ERPF) by about 20%. No differences between the drugs were observed with regard to their effect on renal haemodynamic parameters. By contrast, nifedipine exerted an inhibitory effect on several proximal tubular transport markers, whereas lisinopril was without effect. The different actions on tubular transport mechanisms exerted by lisinopril and nifedipine may contribute to the observed effect on albumin excretion.

Effects of isradipine on renal function in cyclosporin-treated renal transplanted patients.

The renal effects of the calcium-channel antagonist isradipine were evaluated in cyclosporin A (CsA)-treated renal allografted patients more than 5 months after transplantation. Twelve patients with stable renal function were given placebo for 2 weeks and then isradipine 1.25 mg x 2 for 1 week and 2.5 mg x 2 for the next 3 weeks in an open trial. The CsA dose was unchanged during the study. Isradipine did not interfere with the pharmacokinetics of CsA as both whole blood trough and peak levels were unchanged. Isradipine reduced mean arterial blood pressure (MAP) from 117.0 +/- 1.8 to 106.3 +/- 1.9 mmHg (P less than 0.01). The renal effects were studied during water diuresis 1-3 h after drug intake. Para-aminohippurate clearance (CPAH) increased from 256.4 +/- 21.5 to 291.5 +/- 26.3 ml/min (P less than 0.05), renal vascular resistance was reduced by 21.5% (P less than 0.01) and inulin clearance (CIn) was unchanged. Fractional proximal reabsorption, calculated from lithium clearance (CLi), was reduced by 11.9% (P less than 0.01) by isradipine. Isradipine also reduced proximal reabsorption of sodium (APRNa) and increased distal sodium delivery (DDNa). Distal sodium reabsorption (ADRNa) and free-water clearance (CH2O) were significantly increased. Urinary excretion of enzymes and proteins was unchanged by isradipine.

Verapamil potentiates 4'-epidoxorubicin cytotoxicity in a rat hepatoma cell line and in a substrain resistant to 4'-epidoxorubicin.

We have examined the influence of verapamil (VP) on the in vitro effect of 4'-epidoxorubicin (Epi-A) in a rat hepatocarcinoma cell line (MHlCl) and in an Epi-A resistant substrain. A VP concentration of 500 ng/ml (1.1 mumol/l) markedly potentiated the cytotoxic effect of Epi-A in the parent line. The resistant cells grow at an Epi-A concentration of 7500 ng/ml (12.9 mumol/l). This is approximately 15-fold higher than the concentration tolerated by parental cells. In these cells VP reversed the acquired resistance to Epi-A in a concentration dependent manner; thus, a concentration in the range of 500-750 ng/ml (1.1-1.7 mumol/l) of VP restored the sensitivity to Epi-A in the resistant cells. Our results demonstrate that VP increases the sensitivity to Epi-A in hepatocarcinoma cells never exposed to this drug, as well as in hepatocarcinoma cells with acquired Epi-A resistance.

Pharmacokinetics of oral 6-mercaptopurine: relationship between plasma levels and urine excretion of parent drug.

Plasma levels and cumulative urine excretion of 6-mercaptopurine (6-MP) were measured using a specific and sensitive high-performance liquid chromatographic assay in seven children with acute lymphoblastic leukemia (ALL) as well as in one healthy volunteer. The dose of 6-MP varied in the range of 25-75 mg/m2 of body surface area and was administered with a standard breakfast. A 4- to 11-fold variation between individuals was found in the pharmacokinetic parameters: peak concentration, time to reach peak, area under the plasma concentration-time curve (AUC), and fraction of dose excreted in the urine. Three repeated determinations in one individual revealed that AUC also varied more than sixfold following an overnight fast. In three individuals, the reducing agents glutathione (10 mg/kg) and ascorbic acid (15 mg/kg) were coadministered with 6-MP to evaluate their possible role in the protection of 6-MP from oxidation and degradation in the intestinal lumen. No consistent effect was observed, however, on the AUCs of either of these agents. A clear relationship was found between AUCs and the 24-h urinary excretion of unchanged drug (r = 0.9381), indicating that determinations of 6-MP in the urine may replace the painful procedure of repeated blood sampling. Further studies are necessary to determine the factors contributing to the unpredictable plasma levels following oral doses of 6-MP and to determine the value of pharmacokinetic monitoring in ALL patients.

The influence of partial hepatectomy on the pharmacokinetics of preoperatively injected 4'-epidoxorubicin in rats.

Preoperative administration of 4'-epidoxorubicin (Epi-A) has been suggested as adjuvant therapy in patients undergoing liver resection for hepatocarcinoma. To assess the influence of partial hepatectomy on the pharmacokinetics of Epi-A, an experimental study in rats was undertaken in which 5 mg/kg Epi-A was given i.v. 10 min prior to a 2/3 hepatic resection or sham operation. Epi-A levels in liver tissue and plasma were determined using a sensitive and specific HPLC method. A marked uptake of Epi-A in liver tissue was found at 10 min after injection. The partially hepatectomized rats showed a 2-fold increase in AUC between 4 and 72 h as compared with the sham-operated controls. The terminal half-life from 24 to 72 h was not significantly changed by the partial hepatectomy. The plasma binding of Epi-A was measured at 4 h post-surgery. The fraction of unbound Epi-A was 0.16 in partially hepatectomized animals and 0.20 in sham-operated rats. The results indicate that when Epi-A is given prior to liver resection, a dose reduction might be necessary to avoid increased side effects due to the rise in AUC.

Protective function of metallothionein against certain anticancer agents.

The possible role of metallothioneins (MT) in the cellular protection against ionizing radiation and alkylating agent cytotoxicity has been investigated by the use of cultured cells with either low or high levels of MT. The cytotoxic activity of the drugs cis-dichlorodiammineplatinum (cis-DDP), chlorambucil and prednimustine was reduced 1,5-3 fold in cells containing high levels of MT, and furthermore about 70% of platinum and 30% chlorambucil was shown to be associated with the protein. Increased resistance was also demonstrated in MT-rich cells during exposure to ionizing radiation. Evidence that MT also may play a role in vivo as a resistance mechanism, was provided in studies of tumors derived from either the MT-rich or MT-poor cells which had been inoculated in nude mice. During treatment of the animals with cis-DDP, the MT-rich tumors exhibited a significant degree of resistance. These results indicate that MT may play an important role in the intrinsic protection of cells against these agents, and raises the question whether it also may be a factor in the acquired resistance of tumors against chemotherapy.

Chemosensitivity measurements of human tumour cells by soft agar assays are influenced by the culture conditions.

To investigate the influence of culture conditions on the in vitro responses of tumour cells to anticancer drugs, the sensitivities observed with the soft agar methods of Hamburger & Salmon (1977) (H-S) and of Courtenay & Mills (1978) (C-M) were compared. In all cases the ID50 values were determined from dose-response curves. Six human tumour cell lines exposed to 10 different agents, and 9 patients' melanomas exposed to 5 different agents, were examined. In the studies of cell lines the H-S method gave higher sensitivity values than the C-M method in 38 out of 52 cases, whereas in 14 cases the results were the same. In the patients' tumours the H-S method gave higher sensitivity in 21 of 35 cases, equal sensitivity in 11, and lower sensitivity in 3 cases. In many instances the ID50 values obtained with the two test systems differed by factors of 10 or more, both in the case of cell lines and tumour specimens. Systematic alterations in the culture conditions indicated that the presence or absence of rat erythrocytes is the most important factor responsible for the differences observed. Also, other factors, such as supplements (in the H-S method) and the use of different serum types, appeared to influence both colony growth and chemosensitivity.