A comparison of the influence of La3+, D600 and gentamicin on frequency-force relationships in isolated myocardium

Extra- and intracellular lanthanum: modified calcium distribution, inward currents and contractility in guinea pig ventricular preparations

In guinea pig ventricular strips and isolated cells, 0.1 mM LaCl3 blocks contractility and shortens the action potential (AP) in less than 2 min ("early La-effect"). After 30 min, it prolongs the APs which trigger slow contractions ("late La-effect"). These results confirm earlier reports. X-ray microprobe analysis shows that La initially displaces only a small fraction of that Ca which is superficially bound to the sarcolemma. But, since this Ca is completely removed by Ca-free solutions within 2 min, we suggest that La blocks contractility not by displacing superficial Ca but by blocking the Ca inward current iCa. Blocking of iCa is analyzed with voltage clamp experiments. It is not La-specific, and can also be observed with other calcium channel blockers as well. When iCa has been blocked, the membrane can still generate 100-200 ms long plateaus via the sodium inward current iNa. During the late La-effect, the cells internalize La. Intracellular La is detected by x-ray microprobe analysis in cryosections of frozen muscles and as La-precipitates in EM images from freeze substituted preparations. Simultaneously, the cytosol gains Na and Ca, but the plasmalemmal and sarcoplasmic reticulum (SR) membranes are no longer occupied by Ca but by La. The late La-effect on the prolongation of the AP is La-specific. In the absence of extracellular La, it can be induced by pressure injection of La into the cytosol. The long APs are based on an additional La, it can be induced by pressure injection of La into the cytosol. The long APs are based on an additional inward current which is insensitive to Ca-removal, is inactivated by holding potentials of -40 mV, and is TTX-sensitive. We suggest that the current flows through a fraction of original Na-channels that is modified by i.c. La with respect to inactivation and selectivity. We attribute the late re-occurrence of contractility to activator Ca entering from the bath. Ca-entry might be mediated via enhanced Na/Ca-exchange whose rate is increased by the i.c. Na-load. In addition, Ca may enter through the La-modified Na-channels due to their impaired selectivity. Since i.c. La is known to interfere with the Ca-sequestration by the SR, it is expected to impair relaxation.

Inotropic, chronotropic and coronary vasodilator potency of forskolin

The cardiodynamic profile of forskolin, a direct activator of adenylate cyclase, was examined in an isovolumic left ventricular (LV) preparation of coronary-perfused guinea-pig hearts. Forskolin consistently induced concentration-dependent increases in LV systolic pressure development, increases in spontaneous beating frequency and decreases in coronary vascular resistance. However, the forskolin concentration required for one-half of the maximal effect (EC50) for coronary vasodilation (3.8 +/- 0.6 X 10(-8) M, n = 6) was 7- to 10-fold less than the EC50 for positive chronotropy (29.3 +/- 4.6 X 10(-8) M, P less than 0.01) and 17- to 20-fold less than the EC50 for positive inotropy (66.3 +/- 7.4 X 10(-8) M, P less than 0.001). The inotropic response to forskolin was not secondary to the concomitant increase in bearing frequency. Also, the coronary vasodilator response persisted in K+-depolarized non-beating hearts, and therefore did not depend on endogeneous coronary vascular autoregulation secondary to increased myocardial oxygen demand. We conclude that forskolin induces direct pharmacologic effects in ventricular muscle, pacemaker cells and the coronary vasculature, but add that the coronary vasculature is over one order of magnitude more sensitive to forskolin than is ventricular muscle. Perhaps adenylate cyclase systems in the heart exist as different subtypes with different affinity characteristics for forskolin-like drugs.

A study of the mode and site of action of capsaicin in guinea-pig heart and rat uterus

Capsaicin (CAP) has been shown to exert a selective neurotoxic effect on peptidergic primary afferent neurons. The effect of CAP on the isolated guinea-pig left auricle and papillary muscle preparations and on the isolated rat uterus was used to elucidate its mode and site of action with regard to cellular Ca2+ utilization. In the electrically driven left auricle CAP first increased and then decreased the size of the contractions while in the electrically driven papillary muscle CAP caused only a decrease in the contractions. Electrophysiological measurements showed that the initial increase in contraction size coincided with a decrease in the upstroke velocity of the action potential. This membrane-stabilizing effect of CAP seemed also responsible for the decrease in contractile activity. The positive inotropic effect of CAP on the left auricle was concentration-dependent (0.03-6.5 microM). The positive inotropic effect of 0.33 microM CAP was reproducible at intervals of 15 min, whereas tachyphylaxis developed at shorter intervals or higher concentrations of CAP. The percent increase in the size of contractions by 0.33 microM CAP was smaller when [Ca2+]e was doubled but larger when 2.2 microM verapamil or 0.1 mM La3+ was present. The increase in contractions by 0.4-400 microM isoproterenol was greatly reduced by 0.33 microM CAP in a noncompetitive manner. The positive inotropic effect of 2.9 microM glucagon was also inhibited by 0.33 microM CAP. In the isolated anoestrous rat uterus 0.03-3.3 microM CAP caused a transient inhibition of the spontaneous contractions similarly to the effect of 2.2 microM verapamil.(ABSTRACT TRUNCATED AT 250 WORDS)