Search

Spontaneous electrical activity and indo 1 fluorescence ratios were recorded simultaneously in cultured pacemaker cells isolated from the rabbit sinoatrial node. Ryanodine (10 μM) reduced the amplitude of action potential-induced intracellular Ca2+([Formula: see text]) transients by 19 ± 3%, increased the time constant for their decay by 51 ± 5%, and slowed spontaneous firing by 32 ± 3%. 1,2-Bis(2-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM; 25 μM) inhibited the [Formula: see text] transients and slowed spontaneous firing by 28 ± 4%. Ryanodine did not alter hyperpolarization-activated or time-independent inward current, but it reduced the sum of L- and T-type Ca2+ currents ( I Ca,L and I Ca,T) in both the presence and absence of BAPTA-AM. In contrast, I Ca,L was unchanged by ryanodine. Slow inward current tails, presumed to be Na/Ca exchange current ( I Na/Ca), were abolished by BAPTA or ryanodine. The results suggest that a decrement of I Ca,T, due to reduction of the intracellular Ca2+ concentration or a direct effect of ryanodine on T-type Ca2+channels, contributes to the negative chronotropic effect. Another possibility, based primarily on theory and results in other preparations, is that a reduction of I Na/Ca, as a consequence of the smaller action potential-induced[Formula: see text] transients, contributes to the effect of ryanodine.

The negative chronotropic effect of acetylcholine (ACh) on the sinoatrial node fades in the continuous presence of ACh as a result of desensitization. We have investigated the mechanism underlying desensitization in single rabbit sinoatrial node cells using the whole cell patch clamp technique. The negative chronotropic effect resulting from the injection of a constant hyperpolarizing current faded. ACh activated an inwardly rectifying potassium current (iK,ACh), which faded in the continuous presence of ACh. ACh had no effect on “basal” L-type calcium current (iCa), but ACh decreased iCa, which had been potentiated by isoprenaline. This effect did not fade during a 2-min exposure to ACh. ACh decreased the hyperpolarization-activated current (i(f)). This effect again did not fade. These results suggest that desensitization of the negative chronotropic response to ACh is, in part, the result of the membrane hyperpolarization and, in part, the result of the fade of iK,ACh. These results also suggest that, whereas the activation of potassium current by ACh rapidly fades, the effects resulting from the inhibition of adenylate cyclase do not.

We have studied delayed rectifier K+ currents (IK) in cells isolated from the sinoatrial node (SAN) region of the guinea pig. Using whole cell patch-clamp procedures, we measured the voltage dependence of IK activation and IK kinetics and the IK equilibrium potential in 4.8 mM extracellular K concentration solutions. Experiments were designed to contrast properties of guinea pig SAN IK with those of IK recorded from SAN cells of the rabbit. We find that guinea pig SAN IK differs from IK recorded from single rabbit SAN cells in its activation threshold, and in the absence of inactivation of whole cell currents recorded over a wide voltage range. These results, along with the relative insensitivity of guinea pig SAN IK to E-4031 and lanthanum, suggest that under our experimental conditions, a strongly rectifying IK component (IK,r) is not the major component of delayed rectification in the guinea pig SAN, as it appears to be in SAN cells of the rabbit.

The dynamic effects of 1) brief bursts of vagal stimuli and 2) changing heart periods (A-A interval) on the atrioventricular conduction time (AVCT) were studied in open-chest, anesthetized (pentobarbital, 30 mg/kg) dogs (n = 8) under beta-adrenergic blockade (propranolol, 1 mg/kg). Four different atrial and/or vagal stimulation paradigms were used to quantitate the interactions between these two separate effects. We measured the AVCT responses to identical vagal stimuli both when the heart was paced and when it was not paced. When the heart was not paced, we collected and stored in computer memory the sequences of dynamically changing heart periods that followed each vagal stimulus burst. Later, without vagal stimuli, we played these sequences of changing intervals back into the atrial pacing electrodes and measured the subsequent AVCT responses to those changes of heart period alone in the absence of vagal activity. We also measured the AVCT responses when we played these intervals back into the atrial pacing electrodes but when identical vagal stimuli were simultaneously applied, so as to absolutely fix the site of atrial activation. We found that these two separate effects of vagal stimuli and changing A-A intervals on AVCT were not linearly additive. For example, the response of the atrioventricular (AV) node to the two combined effects was less than the linear sum of the AV response to the two separate perturbations. Our results further suggest strongly that shifts in the pacemaker site may not be an important mechanism for the paradoxical response of the AV node to vagal stimulation under the present experimental conditions.