Automated measurement of amylase isoenzymes by a double kinetic assay with "blocked" beta-2-chloro-4-nitrophenyl maltopentaoside as substrate and with wheat germ inhibitor

We evaluated the enzymic mechanism by which 3-keto butylidene-beta-2-chloro-4-nitrophenyl maltopentaoside (3KB-G5-CNP) serves as a substrate for serum pancreatic (p-) and salivary (s-) amylases. In aliquots of the reaction mixture, three kinds of beta-2-chloro-4-nitrophenyl oligosaccharides (glucose, maltoside, and maltotrioside) were separated from the substrate by high-performance liquid chromatography. Both isoenzymes behaved nearly identically and produced almost the same products. We automated a double kinetic procedure for determining total (t-) and p-amylase with use of a selective inhibitor from wheat germ in a single channel on the Hitachi 7050 analyzer. Within- and between-run CVs were, respectively, 0.5% and 1.7% for t-amylase (240 U/L), and 0.7% and 2.3% for p-amylase (230 U/L). The test results varied linearly with concentrations up to approximately 2000 U/L for t- and p-amylase activities. p/s ratios varied from 0.2 to 5.0. Results correlated well with those obtained by the monoclonal inhibition method (r = 0.992).

Automated measurement of amylase isoenzymes by a double kinetic assay with "blocked" beta-2-chloro-4-nitrophenyl maltopentaoside as substrate and with wheat germ inhibitor

We evaluated the enzymic mechanism by which 3-keto butylidene-beta-2-chloro-4-nitrophenyl maltopentaoside (3KB-G5-CNP) serves as a substrate for serum pancreatic (p-) and salivary (s-) amylases. In aliquots of the reaction mixture, three kinds of beta-2-chloro-4-nitrophenyl oligosaccharides (glucose, maltoside, and maltotrioside) were separated from the substrate by high-performance liquid chromatography. Both isoenzymes behaved nearly identically and produced almost the same products. We automated a double kinetic procedure for determining total (t-) and p-amylase with use of a selective inhibitor from wheat germ in a single channel on the Hitachi 7050 analyzer. Within- and between-run CVs were, respectively, 0.5% and 1.7% for t-amylase (240 U/L), and 0.7% and 2.3% for p-amylase (230 U/L). The test results varied linearly with concentrations up to approximately 2000 U/L for t- and p-amylase activities. p/s ratios varied from 0.2 to 5.0. Results correlated well with those obtained by the monoclonal inhibition method (r = 0.992).

Quantitation of serum apolipoprotein A-I, A-II, B, C-II, C-III and E in healthy Japanese by turbidimetric immunoassay: reference values, and age- and sex-related differences

Serum apolipoproteins (Apo) A-I, A-II, B, C-II, C-III and E were determined in healthy Japanese subjects (male 1,603, female 1,800, aged 4-95 yr) by the turbidimetric immunoassay, with six kinds of automated instruments, and the commercial reagent kits with standards. There was a high degree of interlaboratory comparability of analytical values among the 15 participating laboratories. The reference values were calculated for adult males and females (male 677, female 467, aged 21-60 yr). Apo A-I and E levels were significantly higher and Apo C-II and C-III were significantly lower in females than in males. Furthermore, serum Apo A-I and A-II tended to decrease, and Apo B to increase with age. Apo C-II, C-III and E tended to decrease after 60 yr of age.

[Changes in serum cholesterol levels in Japan]

Serum cholesterol levels in Japanese people have increased after World War II. The national surveys by Research Committee demonstrated an increase of approx. 10 mg/dl of serum cholesterol from 1960 to 1980 in every years of age. An increase of serum cholesterol is observed in Japanese, whereas a decrease in American during the 20 years. Although the Japanese cholesterol levels are much lower for the older people, those for younger group are higher than American levels. These results bring forward the problems in the incidence of ischemic heart diseases in young Japanese. It is considered that the changes in serum cholesterol levels are due to the changes in the proportion of the nutrients in the Japanese diets; energy from carbohydrates decreased, whereas ones from proteins and fats increased, with no significant changes in total calorie intake. In particular, the intakes of total and animal fats have increased more than 3.5 fold during the period. "Normal" values and ranges in laboratory medicine are generally derived as the mean +/- 2SD by the statistical method. Normal range, however, which are calculated from many laboratory data, have come into questions in case of serum cholesterol. The upper limits of the means +/- 2 SDs are often higher than the values obtained from the clinical and epidemiological findings. It has clarified that the medical cares are necessary even at lower cholesterol levels. Then, "normal" values and ranges must be decided not only by statistics, but by the clinical and epidemiological findings.

Changes in serum and hepatic levels of immunoreactive prolyl hydroxylase in two models of hepatic fibrosis in rats

Changes in serum and hepatic levels of immunoreactive prolyl hydroxylase (IRPH) as well as cellular localization of the enzyme were studied in 2 models of hepatic fibrosis, which was induced in male rats either by subcutaneous administration of CCl4 (Group A) or by intraperitoneal injection of porcine serum (Group B). Hepatic fibrosis appeared at the 8th week in Group A and at the 12th week in Group B, and liver cirrhosis developed at the 16th week in both models. Although tissue contents of hydroxyproline (HP) and IRPH increased in both models, only HP levels correlated with the degree of fibrosis. Serum IRPH levels and serum asparate aminotransferase (AST) activities increased, showing a significant positive correlation, in group A, whereas both remained in a control range in Group B. Moreover, in another model which received a single intraperitoneal injection of CCl4, serum IRPH showed a marked increase and then a rapid decrease in parallel with the change in serum AST. Immunohistochemical analysis also showed a difference between the two fibrosis models: in group A, IRPH was positive mainly in parenchymal cells in the peripheral zone of the pseudolobulus, while in group B the staining was diffuse. These results indicate that the elevation of serum IRPH is, at least in part, due to the parenchymal cell damage, and that IRPH levels should be carefully evaluated when being used as a parameter to estimate the activity of fibrogenesis in the liver.

The critical role of blood from HLA-homozygous donors in fatal transfusion-associated graft-versus-host disease in immunocompetent patients

Fatal transfusion-associated graft-versus-host disease developed in a 69-year-old woman with colon cancer who underwent elective hemicolectomy. During the perioperative period, she was transfused with 4 units of nonirradiated fresh whole blood less than 6 hours after the blood was donated by family members. She was immunocompetent and was not treated with any immunosuppressive agents such as corticosteroids, chemotherapy, or irradiation therapy. The implicated donor was thought to be her daughter, who was homozygous for an HLA haplotype that was shared with the recipient: A24, Bw52, CBL, DR2. This is the most common haplotype in the Japanese population. This case and others in the Japanese literature indicate that the transfusion of fresh, nonirradiated blood that contains immunocompetent lymphocytes and peripheral hematopoietic precursor cells from HLA-homozygous donors can be lethal to the recipient.

Conductance properties of the Na(+)-activated K+ channel in guinea-pig ventricular cells

1. The Na(+)-activated K+ channel current was recorded from inside-out membrane patches excised from single ventricular cells of guinea-pig hearts. 2. The single channel current-voltage relations showed inward-going rectification with an asymptotic conductance of 180-210 pS for the inward current at 150 mM [K+]o, when [K+]i was changed between 5.4 and 150 mM. The reversal potential indicated the PNa/PK of about 0.02. 3. The amplitude of outward current was reduced by increasing [Mg2+]i or [Na+]i, but no obvious blocking noise was recorded. The outward current, which remained shortly after quick removal of both [Na+]i and [Mg2+]i, revealed an ohmic conductance of the K+ channel. 4. The [Mg2+]i and [Na+]i block was increased e-fold by depolarizing the membrane by 49 mV, while the inward current was not blocked. 5. The Na(+)-activated K+ channel showed frequent subconductance levels. The variance-mean analysis resolved at least ten major sublevels. The density distribution of the sublevels were measured by composing the conventional amplitude histogram, excluding clear closed state currents, and then dividing the histogram into five segments. The probability of staying in each segment (Pn) was almost always voltage independent, and the grand averages were P1 = 9.5 +/- 5.9%, P2 = 6.3 +/- 2.1%, P3 = 4.2 +/- 1.8%, P4 = 7.8 +/- 2.5%, and P5 = 39.3 +/- 5.6%, from the lowest segment, respectively. 6. The values of Pn in partially blocked conditions by Na+ and Mg2+ (outward current) were not clearly different from those without any channel block (inward current). The values of Pn, measured before and after applying Ba2+ in the pipette, were also very similar. 7. The above findings indicate that the inward-going rectification of the Na(+)-activated K+ channel is due to the Na+ and Mg2+ block. The subconductance of the channel is not due to any channel block by Na+ or Mg2+, but may be attributable to multiple open states of a single-barrel channel, which has a large conductance. The channel may be blocked from any open conformation with an equal probability and with very fast kinetics.

Beta-adrenergic and muscarinic regulation of the chloride current in guinea-pig ventricular cells

1. Single guinea-pig ventricular cells were voltage clamped using the patch clamp method combined with the pipette-perfusion technique. The voltage-dependent current systems were mostly blocked, and the background membrane conductance was measured by applying ramp pulses. 2. beta-Adrenergic effectors and related substances such as adrenaline, isoprenaline, forskolin or internal application of cyclic AMP induced a current component which showed a reversal potential near the expected Cl- equilibrium potential as well as an outward rectification in the I-V relation. It is suggested that the activation of this Cl- current was due to phosphorylation of the channel protein or related structure by the cyclic AMP-dependent protein kinase. Coincidentally with the activation of the Cl- current, the membrane capacitance of the cell decreased reversibly. 3. Acetylcholine (ACh) depressed the responses induced by beta-adrenergic stimulation and forskolin, but failed to interfere with the one induced by cyclic AMP. 4. The dose dependence of the Cl- current activation by isoprenaline or forskolin was fitted by the Hill equation, with a coefficient of 1.9 and a half-maximum concentration K 1/2 = 13 nM for isoprenaline, and with a Hill coefficient of 3 and a K 1/2 = 1.2 microM for forskolin. In the presence of 5.5 microM-ACh the dose-response relation shifted to higher doses; K 1/2 was 65 nM for isoprenaline and 3.6 microM for forskolin. 5. Washing out ACh in the presence of isoprenaline frequently caused transient overshoots of the response. When a saturating concentration of isoprenaline was used, this rebound was not observed. 6. The internal application of cyclic GMP enhanced the response of the Cl- current induced by isoprenaline or adrenaline. 7. When cyclic AMP was applied internally, the response was small in most cells. When the cell was superfused with 20 microM-IBMX (3-isobutyl-1-methylxanthine), the Cl- current was consistently induced by the application of cyclic AMP. It is suggested that phosphodiesterase activity strongly buffered the influx of cyclic AMP through the patch pipette tip. 8. We suggest that the compensatory interaction between the beta-adrenergic stimulation and the muscarinic inhibition is at the membrane level, most probably via GTP-binding proteins in activating adenylate cyclase.

Limitations due to unstirred layers in measuring channel response of excised membrane patch using rapid solution exchange methods

The Ca(2+)-activated K+ channel was activated by a step increase in the intracellular Ca2+ concentration ([Ca2+]i) using the oil-gate concentration jump method in hippocampal neurons of rat. Limitations due to the unstirred volume between the excised inside-out membrane and the tip opening were examined.

Selective modulation of the ATP-sensitive K+ channel by nicorandil in guinea-pig cardiac cell membrane

Effects of a vasodilator, nicorandil (2-nicotinamidoethyl nitrate) on four kinds for cardiac K+ channels were investigated in guinea pig ventricular and atrial cells using inside-out patch recording combined with "oil-gate" concentration jump method. Nicorandil of 300 mumols/l failed to affect the inward-rectifier K+ channel and the Na(+)-activated K+ channel. The open probability of the muscarinic K+ channel, when activated by the application of GTP, was not changed by the drug. Nicorandil selectively increased the open probability of the ATP-sensitive K+ channel that was partly suppressed by intracellular ATP. The median effective concentration (EC50) of nicorandil was 74 mumols/l and Hill coefficient was 1.32 in the concentration-open probability relationship. The closing rate of the K+ channel by ATP was markedly delayed by the drug, whereas the open rate on removal of ATP was scarcely affected. Nicorandil had only little effect on this channel after run-down. It was concluded that nicorandil selectively activates the ATP-sensitive K+ channel mainly by modulating the ATP-dependent gate.

Inhibitory effect of acyclic retinoid (polyprenoic acid) on the secretion of alpha-fetoprotein in CCl4-treated rats

A study was conducted to examine the inhibitory effect of acyclic retinoid (polyprenoic acid) on the secretion of alpha-fetoprotein (AFP) in rats with chronic liver damage induced by CCl4. Oral administration of the compound brought about a significant reduction of serum AFP levels at the time when liver cirrhosis was formed. Acyclic retinoid also decreased the activities of serum aminotransferases and ornithine carbamyl transferase, while it increased serum albumin levels, demonstrating the reduction of hepatic parenchymal damage. Significant negative correlation was observed between serum AFP and albumin levels. This cytoprotective effect of the retinoid on the parenchymal cell may well be related to the inhibition of the synthesis and/or secretion of AFP. No significant side effect was observed, despite a long-term administration of the compound. The present finding will provide a potential scope for the future use of acyclic retinoid for the treatment of chronic liver damage.

Reduction of serum lipoprotein(a) levels in hyperlipidaemic patients with alpha-tocopheryl nicotinate

The effect of low dose (600 mg/day) alpha-tocopheryl nicotinate on serum lipoprotein(a) (Lp(a] concentration was studied in 28 hyperlipidaemic patients. Serum lipids, lipoproteins and apolipoproteins, except for Lp(a), tended to increase after treatment. In particular, the changes in HDL-cholesterol and apo C-II levels were statistically significant. On the other hand, serum Lp(a) levels in all patients decreased significantly after 2 months of treatment. Furthermore, no difference between before and after treatment was observed in the group with initial Lp(a) levels less than 18 mg/dl, whereas Lp(a) concentrations decreased significantly after treatment in the group with levels greater than or equal to 18 mg/dl. The effects of probucol and alpha-tocopheryl nicotinate on serum Lp(a), total cholesterol and HDL-cholesterol were entirely different. Possible mechanisms of alpha-tocopheryl nicotinate on serum Lp(a) and lipoprotein metabolism are discussed.

Maximum open probability of single Na+ channels during depolarization in guinea-pig cardiac cells

Single Na+ channel currents were recorded from guinea-pig ventricular cells in cell-attached patches. The ensemble average current (I) of multi-channel recordings was used to calculate the variance (sigma 2) of current fluctuations around the mean in individual current recordings. The relationship between sigma 2/I and I was linear and allowed estimation of the number of functional channels in the patch of membrane. The unitary amplitude of channel current obtained from the relation sigma 2/I-I was in agreement with that obtained directly by measuring the original records. The number of channels determined at different depolarizing pulses was almost constant in a given patch. The value was nearly equal to that of the maximum current, measured at high depolarizing potentials when most channels are open, divided by the unitary current. The open probability of the channels at the peak time of mean current was calculated based on the estimated number of channels. It increased with increasing depolarization and saturated at about 0.6 at test potentials above -20 mV. The inactivation time-course of the mean current was fitted by a sum of two exponentials. The current amplitude extrapolated to time zero was much larger than the current which could be generated by all channels. This indicates that the inactivation of the Na+ channel develops with delay after the onset of depolarization. The finding is in agreement with a model in which the inactivation rate is accelerated with activation of the Na+ channel.

Chloride-sensitive nature of the adrenaline-induced current in guinea-pig cardiac myocytes

1. Ionic selectivity of an adrenaline-induced current was investigated in single guinea-pig ventricular cells by recording whole-cell currents using the patch clamp technique combined with internal perfusion. Other ionic currents and exchange currents known in ventricular cells were suppressed by appropriate inhibitors and the adrenaline-induced current was defined as a difference between currents obtained in the presence and absence of adrenaline. 2. The adrenaline-induced current was time independent and its I-V relation showed saturation of the inward current in the negative voltage range. 3. The reversal potential was approximately -20 mV with 140 mM-NaCl external solution and Cs(+)-rich internal solution containing 51 mM-Cl-. Replacing Na+ with various monovalent and divalent cations (Li+, K+, Rb+, Cs+, Ca2+, Sr2+ and Ba2+) produced no appreciable change in the reversal potential. 4. Varying the external Cl- concentration ([Cl-]o) in exchange for aspartate or benzenesulphonate greatly changed the reversal potential. The relationship between the reversal potential and log[Cl-]o indicated a slope of 59.5 or 53.6 mV per tenfold change in [Cl-]o in the presence of 51 or 102 mM-Cl- in the internal solution, respectively. 5. Anion substitutions did not appreciably affect the I-V relation before application of adrenaline, suggesting that the cell membrane had a low Cl- conductance in the control state. 6. 4.4'-Dinitrostilbene-2-2'-disulphonic acid (DNDS: 1-10 mM), a specific inhibitor of membrane chloride permeability, depressed the adrenaline-induced current without changing the reversal potential. 7. The results suggest strongly that the adrenaline-induced current is carried mainly by Cl-. However, the development of this current appears to depend also on external cations, since the magnitude of the adrenaline response varied depending on the external cations species, with no response in Tris-HCl or TEA-Cl solution. The external cations may facilitate the adrenaline response with a sequence of efficacy of Na+ greater than K+, Rb+ greater than Cs+, Li+, divalent cations.

[Evaluation of the commercial ELISA method for lipoprotein(a) determination and interference by plasminogen]

We evaluated the "Tint Eliza (Bio pool, Co., Sweden)", based on an ELISA, for the determination of serum lipoprotein(a) [Lp(a)], and the immunological interference by plasminogen on the assay. Results obtained by the present assay were compared with those by our polyclonal ELISA method. The standard curve for this assay was linear to 60 mg/dl of Lp(a). Correlation with results by our method (x) was excellent: y = 0.94x + 1.2 mg/dl (n = 46, r = 0.986, Syx = 2.3). Within-run CVs for the method were 7.3, 4.8 and 4.5% at mean Lp(a) concentration of 4.0, 11.4 and 38.6 mg/dl, respectively. Detection limits were 0.6 mg/dl for Lp(a) in 1:15, 606 diluted sample. Immunological cross-reactivity of anti-Lp(a) antibody used in the present assay was observed in the immunoblot with plasminogen. In recommended procedure the specificity of Lp(a) sufficed to distinguish the cross-reactivity of plasminogen. Mean and median values of 148 blood donors were 15.5 and 11.5 mg/dl, respectively, and were compared well with those by our method.

[Effects of age and gender on serum levels of apolipoprotein A-I, A-II, B, C-II, C-III and E, measured by turbidimetric immunoassay, in normolipidemic Japanese elderly]

Accumulating evidence has suggested significant roles of apolipoproteins in lipoprotein metabolism. Therefore it is important to measure serum levels of apolipoproteins to understand the pathophysiology of lipid metabolism. The newly developed turbidimetric immunoassay (TIA) method for the determination of serum apolipoprotein concentration is the first method which can be employed with autoanalyzer, and is expected to be used widely. The standard values of serum concentrations of apolipoproteins measured by the TIA method for Japanese adults and children have been reported recently, but not yet for the elderly. Therefore, we investigated serum concentrations of apolipoproteins (A-I, A-II, B, C-II, C-III and E) in healthy normolipidemic elderly, aged 65-99 years, in order to obtain the standard values of apolipoproteins for the elderly. The subjects consisted of 84 males and 91 females. They had normal serum values for total cholesterol (TC) (120-220 mg/dl) and triglyceride (TG) (50-150 mg/dl). No one demonstrated clinical, biochemical or haematological evidence of cardiovascular, peripheral-vascular, cerebrovascular, hepatic, renal or endocrine disorders. Between-assay and intra-assay coefficient variations were approximately 5% and approximately 1.5%, respectively, in all apolipoproteins examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effect of acyclic retinoid (polyprenoic acid) on hepatic fibrosis in CCl4-treated rats

A study was conducted to examine the inhibitory effect of acyclic retinoid (polyprenoic acid) on the development of hepatic fibrosis induced by CCl4 in rats. Oral administration of the compound brought about a significant reduction in both serum and tissue levels of immunoreactive prolyl hydroxylase, a key enzyme of collagen formation. The result indicated that the rate of collagen synthesis in the liver was decreased which was consistent with histological findings. Acyclic retinoid also decreased both AST and ALT activities in serum, demonstrating the reduction in hepatic parenchymal damage. This cytoprotective effect on parenchymal cells may be related, at least in part, to inhibition of hepatic fibrosis. No significant side effects were observed, despite a long-term administration of the acyclic retinoid. The present findings suggest the potential scope of therapy of hepatic fibrosis by retinoid.

Plasma retinol transport system and taste acuity in patients with obstructive jaundice

A study was conducted to elucidate the correlation between the plasma retinol transport system and taste acuity in patients with obstructive jaundice (OJ). Plasma levels of retinol, retinol-binding protein (RBP), transthyretin (TTR) and holo-RBP (retinol-RBP complex unbound to TTR), as well as the threshold of taste acuity, were determined in 8 cases with OJ (6 cases with common bile duct cancer and 2 cases with pancreas head cancer) and in 20 apparently healthy volunteers. These parameters were also measured serially in patients with OJ before and after percutaneous transhepatic biliary drainage (PTBD). Plasma levels of retinol, RBP and TTR were significantly decreased in every patients with OJ as compared with healthy controls, whereas changes in holo-RBP levels were not consistent. Taste acuity was found to be reduced in patients with decreased holo-RBP levels, while the acuity was preserved well in patients whose holo-RBP levels remained normal. Impaired taste acuity was rapidly improved in every patients after treatment with PTBD, significantly correlating with the recovery of plasma RBP and holo-RBP levels, and with the reduction of plasma total bilirubin levels. These results suggest that taste acuity is affected by the plasma retinol transport system, including serum levels of holo-RBP, from which retinol is delivered, presumably through a receptor-mediated manner, to the taste buds.

Negative shift of cardiac Na+ channel kinetics in cell-attached patch recordings

Na+ channel kinetics were studied by recording single-channel currents in the cell-attached patch configuration of the patch-clamp technique in single ventricular cells isolated from guinea pig hearts. The inactivation time course of ensemble currents was accelerated, and the peak amplitude increased temporarily and then decreased within a few minutes after the gigaohm seal formation. After reaching a new steady state, the inactivation-voltage relation was found to have shifted to more negative potentials. The potential of half-maximal inactivation was more negative by 20-31 mV from the resting potential or between -96 and -112 mV. The voltage dependency of the channel activation also shifted. Although the cell membrane was depolarized using the whole cell patch-clamp electrode and single-channel currents were recorded with an independent cell-attached electrode, the shift of the inactivation curve was also evident. Complete removal of Ca2+ using 5 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid in the pipette solution failed to prevent the shift. Increasing Ca2+ to 10 mM, however, reduced magnitude of the shift significantly. Involvement of an increased membrane fluidity and surface potential of the glass pipette to the shift is discussed.

ATP-dependent decay and recovery of K+ channels in guinea pig cardiac myocytes

ATP-dependent decay and recovery of the inward rectifier and ATP-sensitive K+ channels were investigated using inside-out patch recording in cardiac myocytes. The solution facing the inner side of the membrane was instantaneously changed with the oil-gate concentration jump method. Both channels were decayed by removing ATP and were recovered by reapplying ATP. The coexistence of Mg2+ was required for the recovery. 5'-Adenylylimidodiphosphate failed to reverse the ATP-dependent decay. The cumulative histograms of survival time and recovery time, obtained from the inward rectifier K+ channel, showed a single exponential distribution, time constants of which were 55 and 43 s, respectively. The time-dependent nature of decay and recovery was also confirmed in the ATP-sensitive K+ channel. The findings indicated that intracellular ATP is one of the factors that determines the activity of the K+ channels. It is most probable that phosphorylation of channel molecules is essential for maintaining the K+ channel in an operative state.

Li+ inhibition of membrane current responses to epinephrine in guinea-pig ventricular cells

Membrane currents of guinea-pig ventricular myocytes were recorded using the whole-cell voltage clamp method. The epinephrine-induced increase in Ca2+ current (2.9 +/- 0.5 times control) was reduced (1.8 +/- 0.3 times) by replacing Na+ with Li+ in the bathing solution. In addition, 0.5 microM epinephrine increased a time-independent membrane conductance in the Na+ external solution, having a reversal potential of -19 +/- 3 mV (epinephrine-induced current). In the Li+ external solution, however, 0.5 microM epinephrine failed to induce the epinephrine-induced current. The findings are consistent with the reported Li+ inhibition of GTP-binding protein and/or adenylate cyclase.

The Mg2+ block and intrinsic gating underlying inward rectification of the K+ current in guinea-pig cardiac myocytes

1. The blockade by Mg2+ and intrinsic gating of the channel, which underlie the rectification of the inward rectifier K+ current, was investigated using the oil-gap voltage clamp method in isolated guinea-pig ventricular cells. 2. The inward rectifier K+ current was isolated by subtracting trans-gap currents recorded at an extracellular K+ concentration ([K+]o) of 0 mM from those obtained at 14 mM [K+]o in the presence of a given concentration of intracellular Mg2+ ([Mg2+]i). The reversal potential (V0) of the difference current was near the equilibrium potential for K+ (EK). 3. On repolarization across EK, the inward rectifier K+ current showed a rapid exponential increase. The time constant decreased with increasing hyperpolarization, but it was independent of both [Mg2+]i and the preceding depolarization. 4. When the pre-pulse potential was made progressively positive between V0-20 and V0 + 30 mV, the amplitude of the time-dependent component became larger and the preceding current jump decreased at any [Mg2+]i. With pre-pulses more positive than V0 + 40 mV, the time-dependent component started from almost the zero current level at 2 microM [Mg2+]i. At higher [Mg2+]i (350, 500 and 3000 microM), however, the time-dependent component became smaller as the pre-pulse potential was made more positive than V0 + 40 mV. 5. When the membrane was depolarized from a potential of full activation at 2 microM [Mg2+]i, the initial jump in the outward current was ohmic and was followed by an exponential decay. The time-dependent component of the inward current, recorded on repolarization after increasing durations of the preceding depolarization, developed as the outward current decayed. The time constants of both processes were in good agreement. 6. At 500 microM [Mg2+]i, the outward current on depolarization was instantaneously rectified. The time-dependent component recorded on repolarization developed with prolongation of the pre-pulse with a time course slower than at 2 microM [Mg2+]i. The envelope time course became slower as the potential of the depolarization became more positive. 7. Lowering the temperature from 23 to 15 degrees C slowed the time-dependent current with an apparent Q10 of about 3.5 at V0. 8. Based on the experimental data, kinetic parameters were estimated for a model of Mg2+ block, which well simulated the inward-going rectification of the K+ current. 9. It is concluded that the instantaneous inward rectification on depolarization is due to the Mg2+ block at physiological [Mg2+]i.(ABSTRACT TRUNCATED AT 400 WORDS)