Direct activation of Ca²⁺ channels by palmitoyl carnitine, a putative endogenous ligand

Abstract

Palmitoyl carnitine, a lipid metabolite which accumulates in cytoplasmic membranes during ischaemia, has been shown to resemble the Ca2+ channel activator, Bay K 8644, in K+‐depolarized smooth muscle. Palmitoyl carnitine caused concentration‐dependent (1–1000 μmol l−1) augmentations in the sensitivity to Ca2+ of K+‐depolarized taenia preparations from the guinea‐pig caecum. The (±)‐isomer was equieffective with the (–)‐isomer, whereas carnitine was ineffective and palmitic acid relaxed the tissues. The shift to the left of Ca2+ concentration‐response curves induced by palmitoyl carnitine (100 μmol l−1) was additive with that of Bay K 8644 (1 μmol l−1). The interactions of palmitoyl carnitine with the different classes of calcium‐antagonist were similar to those seen with Bay K 8644. Schild plots of the calcium‐antagonist effects of nifedipine were shifted to the right following preincubation of the taenia with palmitoyl carnitine (30–300 μmol l−1). The inhibitory effects of verapamil were especially sensitive to palmitoyl carnitine (100 μmol l−1). Whereas the potency of diltiazem as a calcium‐antagonist was reduced by palmitoyl carnitine (100 μmol l−1), the inhibitory effects of the lipophilic class III calcium‐antagonists, cinnarizine and flunarizine, were entirely resistant to palmitoyl carnitine (100 μmol l−1). Although palmitoyl carnitine has detergent properties in high concentrations and lyses red blood cells, these effects were not Ca2+‐dependent, nor were they modified by calcium‐antagonists. Other detergents did not have selective interactions with Ca2+ channels. Palmitoyl carnitine inhibited [3H]‐nitrendipine, [3H]‐verapamil and [3H]‐diltiazem binding to rat cortical membranes with IC50 values (μmol l−1) of 120 ± 1, 95 ± 17 and 120 ± 15 μmol l−1 respectively. The inhibition showed little temperature‐dependence, in contrast to that of Bay K 8644, except for a small reduction in the IC50 value for [3H]‐verapamil binding at 37°C (42 ± 5 μmol l−1). Palmitoyl carnitine interacted selectively with the Ca2+ channel, in that effects on ligand binding to α‐adrenoceptors, β‐adrenoceptors and 5‐HT1A receptors occurred only at 5–10 fold higher concentrations. It is concluded that palmitoyl carnitine, at concentrations which have previously been shown to occur in the cytoplasm during myocardial ischaemia, may interact directly with Ca2+ channels and may therefore be considered as an endogenous modulator of channel function. The site of action differs from that of other agents.

Prophylaxis of early ventricular fibrillation by inhibition of acylcarnitine accumulation.

“…Recently, we have shown that exogenous LPC can induce delayed afterdepolarizations and triggered rhythms in vitro (36), effects stimulated by catecholamines. Long-chain acylcarnitines can activate calcium channels in cardiac muscle and smooth muscle (7)(8)(9). Accordingly, the antiarrhythmic influence of POCA seen in the present study is likely to involve amelioration of both reentrant and non-reentrant mechanisms responsible for arrhythmogenesis in ischemic myocardium.…”

“…Long-chain acylcarnitines, one class of amphipathic metabolites, activate calcium channels in cardiac (7,8) and smooth muscle myocytes (8,9). Accordingly, they may potentiate the increase in cytosolic calcium associated with arrhythmogenesis in ischemic myocardium (10).…”

l-Carnitine Increases Survival in a Murine Model of Severe Verapamil Toxicity

“…Palmitoyl carnitine and other acylcarnitines have been shown to be endogenous in vitro ligands for calcium channel activation. 18,19 This activation of myocardial calcium channels has been used to explain the positive inotropic effects of palmitoyl carnitine on chick heart cell aggregates. 20 It is therefore possible in our model that the IV L-carnitine is being transported into myocytes and bound to free fatty acids with acetyl-CoA to form higher concentrations of acylcarnitines.…”

International Union of Basic and Clinical Pharmacology. XC. Multisite Pharmacology: Recommendations for the Nomenclature of Receptor Allosterism and Allosteric Ligands

“…This does render screening difficult at times. For example, exquisitely potent compounds acting at one site may appear to have no effect when assessed on a different allosteric, or an orthosteric, site, because the apparent zero effect is a result of negative and positive allosteric effects that are revealed by changing temperature and other experimental variables (Spedding and Mir, 1987); hence the need to define the experimental conditions when quantifying and classifying allosteric compounds. In general, two useful themes in the pharmacological approach to demonstrating allostery are 1) the ability to identify saturability of effect and 2) the ability to demonstrate probe dependence.…”