Search

Although [[beta].sub.2]-adrenoceptors represent 15-25% of [beta]-adrenoceptors in the guinea pig heart, their functionality is controversial. We assessed the inotropic effects of [[beta].sub.2]-adrenoceptor partial agonists in right papillary muscles. Salbutamol induced a small but significant concentration-dependent negative inotropic effect (NIE, -5% at 60 nM) followed by a moderate positive inotropic effect (+ 36% at 6 [micro]M) due to activation of [[beta].sub.1]-adrenoceptors. In the presence of 4 [micro]M atenolol, the concentration-dependent NIE (- 12% at 6 [micro]M) was biphasic, best described by a double logistic equation with respective [EC.sub.50] values of 3 and ~420 nM, and was insensitive to SR59230A. In muscles from pertussis toxin-treated guinea pigs, the salbutamol-induced positive inotropic effect was sensitive to low concentrations of ICI-118551 in an unusual manner. Experiments in reserpinized animals revealed the importance of the phosphorylation-dephosphorylation processes. PKA inhibition reduced and suppressed the effects obtained at low and high concentrations, respectively, indicating that its activation was a prerequisite to the NIE. The effect occurring at nanomolar concentrations depended upon PKA/phosphatidylinositol 3-kinase/ cytosolic phospholipase [A.sub.2] ([cPLA.sub.2]) activations leading to nitric oxide (NO) release via the arachidonic acid/cyclooxygenase pathway. NO release via PKA-dependent phosphorylation of the receptor was responsible for the inotropic effect observed at submicromolar concentrations, which is negatively controlled by [cPLA.sub.2]. The possibility that these effects are due to an equilibrium between different affinity states of the receptor ([G.sub.s]/[G.sub.i] coupled and [G.sub.i] independent with different signaling pathways) that can be displaced by ICI-118551 is discussed. We conclude that [[beta].sub.2]-adrenoceptors are functional in guinea pig heart and can modulate the inotropic state. salbutamol; [beta]-adrenoceptor antagonists; cardiac contractility; [G.sub.s]/ -[G.sub.i] coupling; active conformations

We developed an easy to use and non-invasive method to study sarcomere motion of enzymatically isolated myocytes which can be simultaneously combined with auxotonic force detection, thus being very useful when studying the contractile performance of cardiac cells. This method basically consists in analyzing the periodicity of the cell striation pattern using the Cooley-Tukey fast Fourier transform (FFT) algorithm on a video image of the cell during the course of the experiment. A longitudinal fraction of the cell image is recorded with a CCD TV camera, digitized, then transiently stored on a computer and used to calculate the spectrum corresponding to the distribution of the sarcomere lengths (SL). The method gives a real-time measurement of the most probable value of sarcomere length in one isolated cell with a temporal resolution of 20 ms. When used on a cell attached between two carbon fibers, the auxotonic force developed by the cell upon electrical stimulation can be simultaneously measured together with the SL in various conditions of stretch. Preliminary results have been presented in abstract form (Gannier et al., vol 24, pp. S47, 1992).

1 The effects of 10 mM caffeine (CAF) on intramembrane charge movements (ICM) were studied in isolated guinea-pig ventricular heart cells with the whole-cell patch-clamp technique. 2 In the presence of CAF, the properties (voltage dependence, maximum Q[sub ON] [Q[sub max]], availability with voltage) of Q[sub ON] charge activated from -110 mV were barely affected. Following a 100 ms prepulse to -50 mV to decrease the participation of charges originating from Na channels, the voltage dependence of Q[sub ON] was shifted by 5 mV (negative component) and by 10 mV (positive component) towards negative potentials, and Q[sub max] was depressed by 16.5%. 3 CAF drastically reduced in a time- and voltage-dependent manner Q[sub OFF] on repolarization to -50 mV, the effects being greater at positive potentials. 4 CAF-induced Q[sub OFF] immobilization could be almost entirely removed by repolarization to voltages as negative as -170 mV. In these conditions, the voltage-dependence of Q[sub OFF] (repolarization to +30 to -170 mV) was shifted by 17 mV (negative component) and 30mV (positive component) towards negative potentials, suggesting an interconversion into charge 2. 5 Most of CAF effects were suppressed when the sarcoplasmic reticulum (SR) was not functional or when the cells were loaded with BAPTA-AM. 6 We conclude that CAF effects on ICM are likely due to Ca[sup 2+] ions released from the SR, and which accumulate in the subsarcolemmal fuzzy spaces in the vicinity of the Ca channels. Because CAF effects were more pronounced on Q[sub OFF] than on Q[sub ON] the channels have likely to open before Ca[sup 2+] ions could affect their gating properties. It is speculated that such an effect on gating charges might contribute to the Ca-induced inactivation of the Ca current. [ABSTRACT FROM AUTHOR]

Objective: The aim was to test the hypothesis that in single guinea pig ventricular myocytes a large stretch induced increase in resting calcium was sensitive to the mechanosensitive channel blocker streptomycin. Methods: Carbon fibres were used to stretch cells loaded with the fluorescent calcium indicator indo-1. Force, sarcomere length, and internal calcium activity ([Ca2+]i) were measured. Results: In approximately 60% of the cells studied, a stretch which increased sarcomere length by approximately 6% caused a large increase in [Ca2+]i (up to 60% of the size of a [Ca2+]i transient at 0.25 Hz). When a mixture of antibiotics (streptomycin-penicillin) was used in solutions to isolate and store cells, this phenomenon was never observed (n = 19 cells). Direct application of physiological saline solution (PSS) could not reverse the increase in [Ca2+]i within 60 s of application (n = 7 cells). Direct application of penicillin [1000 IU per 50 ml (40 μM)] reversed the increase in [Ca2+]i within 60 s of application in only 3/7 cells. In contrast direct application of the aminoglycoside antibiotic streptomycin (40 μM) rapidly reversed the large increase in [Ca2+]i induced by stretch in each of 13 cells [within 18(SD 10) s of application]. Acute application of 40 μM streptomycin did not modify L-type Ca2+ currents measured under whole cell patch clamp conditions. Measurement of the resting tension – sarcomere length curves in cells stored in solution containing streptomycin and penicillin revealed two populations of cells on the basis of their stiffness. Conclusions: This stretch induced increase in [Ca2+]i may be associated with stretch activated arrhythmias in the heart. The effects of streptomycin are consistent with its reported inhibitory action on stretch activated channels.Cardiovascular Research 1994;28:1193-1198 [ABSTRACT FROM PUBLISHER]