Tertiapin-Q removes a large and rapidly acting component of vagal slowing of the guinea-pig cardiac pacemaker

Effects of tertiapin-Q and ZD7288 on changes in sinoatrial pacemaker rhythm during vagal stimulation

Heart rate slowing produced by cardiac parasympathetic (vagal) stimulation is thought to be the result of modulation of the acetylcholine-activated K(+) current (IK,ACh) and the pacemaker current (If) in sinoatrial (SAN) pacemaker cells. However, the contribution of these and other ion currents to vagal slowing is controversial. Here, we examined the contributions of IK,ACh and If to vagal slowing in 15 isolated, vagal-innervated preparations of guinea-pig atria, using 300 nM tertiapin-Q (TQ) and 2 μM ZD7288 to obtain full and substantial block of these currents, respectively. Blocking IK,ACh alone reduced atrial rate responses to 10-s trains of regular vagal stimulation (supramaximal stimulation, 2-ms duration, 1-10 Hz) by ~50% (P<0.01; N=11); blocking If alone had no effect (N=7). Blocking both IK,ACh and If produced ~90% reduction (P<0.01; N=4). Atrial cycle length response to a single burst of vagal stimuli (3 stimuli at 50 Hz), delivered at the optimum phase of the cycle was strongly suppressed by blocking IK,ACh (reduced by 98%; P<0.01; N=9), and modestly reduced by blocking If alone (by ~43%; P=0.20; N=6). The response was abolished by combined block of IK,ACh and If (P=0.04; N=4). Our data show that modulation of IK,ACh and If is sufficient to account for all the vagal slowing observed in this preparation. The vagally-induced negative shift in activation potential for If will be opposed by hyperpolarisation of SAN through activation of IK,ACh. Thus removal of IK,ACh by TQ may have exaggerated the overall contribution of If to vagal slowing.

The importance of Ca(2+)-dependent mechanisms for the initiation of the heartbeat

Mechanisms underlying pacemaker activity in the sinus node remain controversial, with some ascribing a dominant role to timing events in the surface membrane (“membrane clock”) and others to uptake and release of calcium from the sarcoplasmic reticulum (SR) (“calcium clock”). Here we discuss recent evidence on mechanisms underlying pacemaker activity with a particular emphasis on the many roles of calcium. There are particular areas of controversy concerning the contribution of calcium spark-like events and the importance of I(f) to spontaneous diastolic depolarisation, though it will be suggested that neither of these is essential for pacemaking. Sodium-calcium exchange (NCX) is most often considered in the context of mediating membrane depolarisation after spark-like events. We present evidence for a broader role of this electrogenic exchanger which need not always depend upon these spark-like events. Short (milliseconds or seconds) and long (minutes) term influences of calcium are discussed including direct regulation of ion channels and NCX, and control of the activity of calcium-dependent enzymes (including CaMKII, AC1, and AC8). The balance between the many contributory factors to pacemaker activity may well alter with experimental and clinical conditions, and potentially redundant mechanisms are desirable to ensure the regular spontaneous heart rate that is essential for life. This review presents evidence that calcium is central to the normal control of pacemaking across a range of temporal scales and seeks to broaden the accepted description of the “calcium clock” to cover these important influences.

The muscarinic-activated potassium channel always participates in vagal slowing of the guinea-pig sinoatrial pacemaker

Controversy persists regarding participation of the muscarinic-activated potassium current (c(KACh)) in small and moderate vagal bradycardia. We investigated this by (i) critical examination of earlier experimental data for mechanisms proposed to operate in modest vagal bradycardia (modulation of I(f) and inhibition of a junctional Na(+) current) and (ii) experiments performed on isolated vagally-innervated guinea-pig atria. In 8 superperfused preparations, 10-s trains of vagal stimulation (1 to 20Hz) produced a bradycardia that ranged from 1 to 80%. Hyperpolarisation of sinoatrial cells accompanied bradycardia in 65/67 observations (linear correlation between bradycardia and increase in maximum diastolic potential (mV)=0.076x%; R(2)=0.57; P<0.001). In bath-mounted preparations single supramaximal stimuli to the vagus immediately and briefly increased pacemaker cycle length in 7 of 18 preparations. This response was eliminated by 300nM tertiapin-Q. Trains of 10 single supramaximal vagal stimuli applied at 1-s intervals caused progressive increase in overall cycle length during the train; immediate and brief increases in cycle length occurred following some stimuli. Immediate brief responses and part of the slower response to the stimulus train were removed by 300nM tertiapin-Q.

SUMMARY

experimental data shows that small and modest vagal bradycardia is accompanied by hyperpolarisation of the pacemaker cell which is severely attenuated by tertiapin-Q. These observations support the idea that activation of I(KACh) occurs at all levels of vagal bradycardia. Contradictory conclusions from earlier studies may be attributed to the nature of experimental models and experimental design.

Tertiapin-Q removes a mechanosensitive component of muscarinic control of the sinoatrial pacemaker in the rat

1. In an isolated right atrial preparation, an increase in right atrial pressure (RAP) produces an increase in atrial rate. This rate response is larger and occurs faster when there is background vagal or muscarinic stimulation. 2. We hypothesized that in the latter situation, an increase in RAP antagonizes the effect of muscarinic stimulation through stretch inactivation of the mechanosensitive muscarinic potassium current I(K,ACh). 3. In two groups of bath-mounted right atria isolated from male Wistar rats (control n = 12; 300 nmol/L tertiapin-Q treated (to block I(K,ACh)) n = 10), we examined the change in atrial rate when RAP was raised from 2 to 8 mmHg; oxotremorine-M (oxo-M; from 10 to 500 nmol/L) was added to incrementally activate muscarinic receptors. 4. In both control and tertiapin-Q-treated groups, oxo-M reduced atrial rate, but its effect was less ( approximately 40-50%) in the latter group (P < 0.001). In control preparations, responses to an increase in RAP became progressively larger and quicker as the concentration of oxo-M was increased, whereas in tertiapin-Q treated preparations oxo-M did not affect either the amplitude or the speed of the response (P < 0.0001 for both). 5. The results support the hypothesis that atrial stretch antagonizes muscarinic slowing by its effect on I(K,ACh). We suggest that through this mechanism, parasympathetic control of heart rate may be modulated continuously by RAP.