Myocardial kinetics of Tc-99m glucarate in low flow, hypoxia, and aglycemia.

BACKGROUND: Technetium-99m glucarate is a myocardial infarct-avid imaging agent. Recent conflicting and inconclusive reports have suggested that the agent may be taken up by ischemic but viable myocardium. The purposes of this study were (1) to determine conclusively whether there is Tc-99m glucarate uptake in ischemic viable myocardium and (2) to investigate the potential mechanisms for such uptake by studying components of ischemia, namely, low flow, hypoxia, and aglycemia. METHODS AND RESULTS: Rat hearts were isolated and perfused in a modified Langendorff preparation with a crytalloid perfusate. Tc-99m glucarate was studied in control (n = 6), low-flow (n = 5), hypoxic (n = 5), and aglycemic (n = 5) conditions. The experimental protocol consisted of 20-minute baseline (12 mL/min flow) and 20-minute treatment (low flow at 1 mL/min, hypoxia, or aglycemia), followed by tracer uptake (20 minute) and washout (20 minutes). Activity was monitored with a sodium iodide detector. The tracer was delivered continuously over a 20-minute uptake period. The injected dose was 150 micro Ci (5.6 MBq). Hemodynamics were monitored throughout. Triphenyltetrazolium chloride staining was used to assess myocardial viability. There was no evidence of myocardial necrosis. Low flow tended to delay tracer uptake compared with control for the first 10 minutes, but this did not reach statistical significance. Low flow increased end fractional retention significantly compared with control (mean +/- SEM, 59.0% +/- 0.9% peak vs 41.2% +/- 1.4%, respectively; P <.05). Hypoxia resulted in a trend toward increased uptake; however, this was significant only at one early time point during the uptake phase. Retention in the hypoxia group was similar to control. Tc-99m glucarate uptake was significantly increased in aglycemia from 16 minutes to peak compared with control (1.36% +/- 0.71% injected dose per gram vs 0.91% +/- 0.37% injected dose per gram, respectively; P <.05). Aglycemia produced significantly higher end fractional retention compared with control (51.6% +/- 1.8% peak vs 41.2% +/- 1.4%, respectively; P <.05). CONCLUSIONS: Tc-99m glucarate myocardial retention is increased in the setting of ischemia, even in the absence of necrosis. This increased retention is not due to hypoxia. Furthermore, the retention is only partially explained by tissue hypoglycemia. Thus low flow per se appears to have a role in this increased retention, probably as a result of delayed flow-dependent washout.

Reduction of NMDA-induced Ca2+ transients by a mu-opioid receptor agonist in dorsal horn neurons.

The effect of DAMGO, a mu-opioid receptor agonist, on intracellular Ca2+ transients evoked by the application of NMDA, was studied in freshly dissociated neurons from the superficial dorsal horn in the spinal cord of young rats. DAMGO (5-10 microns) reduced the amplitude of the Ca2+ transients measured with Fura-2 to 58 +/- 17% of the controls in 41% of the neurons tested. The effect of DAMGO was dose dependent and reversible. The reduction of NMDA-induced Ca2+ transients by DAMGO was prevented by application of the opioid antagonists naloxone (0.1-5 microM) and CTAP (0.2-2 microM). DAMGO also reduced Ca2+ transients induced by high K+ in 29% of the neurons. These data suggest that mu-receptor activation regulates NMDA-induced Ca2+ transients in a complex manner, by reducing both a depolarization-induced component and the NMDA-channel component of this Ca2+ signal.

A patch-clamp study of delayed rectifier currents in skeletal muscle of control and mdx mice.

1. Potassium currents were measured in the extensor digitorum longus muscle of normal and mdx mice, which lack the protein dystrophin, using the cell-attached and inside-out patch clamp techniques, in the presence of asymmetrical K+ concentrations (3 mM in the pipette, 160 mM in the bath). 2. In cell-attached patches, the delayed rectifier was the most commonly found potassium channel, with a density of roughly 8 channels microns-2. Outward macroscopic currents were activated in macropatches depolarized to potentials positive to -60 mV. The probability of opening reached half-maximal values around -40 mV for control patches and -31 mV for patches from mdx mice. 3. Tail currents were linear in the range between -60 and +20 mV, reversing close to -100 mV. The single channel current at 0 mV, estimated from non-stationary analysis of variance, was used in conjunction with the slope of the linear part of the tail current to calculate the single channel conductance, yielding a value of 19 +/- 1 pS. 4. At 0 mV, the delayed rectifier inactivated with two time constants, of 70 +/- 20 ms and 600 +/- 200 ms. Prepulses of 500 ms duration to different potentials produced incomplete inactivation with inactivation reaching 50% of its maximum at -50 mV. 5. Single channel activity was recorded using small pipettes. Both single channel conductance and kinetic behaviour were in agreement with the macroscopic current data. 6. In excised patches, the delayed rectifier current ran down, unmasking other K+ channels. A Ca(2+)-dependent K+ channel of 186 pS (BK-like channel) was found frequently in patches bathed in solutions containing appropriate concentrations of calcium, especially at stronger depolarizations. A K+ channel of 63 pS was unmasked in control excised patches bathed in solutions devoid of ATP. This channel was not observed in patches excised from mdx fibers.

An analysis of the long-lasting after-hyperpolarization of guinea-pig vagal motoneurones.

1. The long-lasting after-hyperpolarization which characterizes the neurones of the dorsal motor nucleus of the vagus in the guinea-pig was studied in vitro. 2. Following a train of action potentials, vagal motoneurones develop a long-lasting after-hyperpolarization. Two different shapes of long-lasting after-hyperpolarization were encountered: an after-hyperpolarization which slowly (0.6-1.2 s) and monotonically developed to peak value; and a second type of long-lasting after-hyperpolarization where the onset of the slow component appears to be masked by an early, relatively fast component. Both shapes of long-lasting after-hyperpolarization depend on Ca2+ influx and increase as a function of the number of action potentials in the train. 3. A novel procedure was used to analyse the ionic processes which underlie the long-lasting after-hyperpolarization. The neuronal responses to a series of long (7 s) hyperpolarizing current pulses during the long-lasting after-hyperpolarization were recorded and the voltage-current curves at 600 different time points along the long-lasting after-hyperpolarization were plotted. The conductance and the reversal potential at each time point were calculated from the slope and the intersection of these curves, respectively. 4. Using this procedure it was found that the long-lasting after-hyperpolarization consists of two conductances that differ in kinetic properties and reversal potential: an early conductance which peaks shortly after the end of the train and decays in a few tenths of seconds (EAHP), and a late conductance which develops slowly (time to peak about 1 s) and decays in 3-8 s (LAHP). The reversal potential for the early conductance is 10 mV more positive than the reversal potential for the late conductance (-84 mV); the latter reversal potential is in agreement with the K+ equilibrium potential. The different shapes of long-lasting after-hyperpolarization can be explained by different ratios of these two conductances. 5. Noradrenaline (10 microM) selectively blocks the late conductance, without an observable effect on the Ca2+ action potential. 6. The behaviour of the noradrenaline-sensitive late conductance was analysed. The amplitude of the conductance change increased sigmoidally as a function of the number of spikes in the train. A log-log plot suggests that at least two Ca2+ ions participate in the opening of a K+ channel. 7. A model that accounts for the slow kinetics of the late conductance was constructed.(ABSTRACT TRUNCATED AT 400 WORDS)