Experimental factors that impact Ca(V)1.2 channel pharmacology—Effects of recording temperature, charge carrier, and quantification of drug effects on the step and ramp currents elicited by the “step-step-ramp” voltage protocol

BACKGROUND AND PURPOSE: Ca(V)1.2 channels contribute to action potential upstroke in pacemaker cells, plateau potential in working myocytes, and initiate excitation-contraction coupling. Understanding drug action on Ca(V)1.2 channels may inform potential impact on cardiac function. However, literature shows large degrees of variability between Ca(V)1.2 pharmacology generated by different laboratories, casting doubt regarding the utility of these data to predict or interpret clinical outcomes. This study examined experimental factors that may impact Ca(V)1.2 pharmacology. EXPERIMENTAL APPROACH: Whole cell recordings were made on Ca(V)1.2 overexpression cells. Current was evoked using a “step-step-ramp” waveform that elicited a step and a ramp current. Experimental factors examined were: 1) near physiological vs. room temperature for recording, 2) drug inhibition of the step vs. the ramp current, and 3) Ca(2+) vs. Ba(2+) as the charge carrier. Eight drugs were studied. KEY RESULTS: Ca(V)1.2 current exhibited prominent rundown, exquisite temperature sensitivity, and required a high degree of series resistance compensation to optimize voltage control. Temperature-dependent effects were examined for verapamil and methadone. Verapamil’s block potency shifted by up to 4X between room to near physiological temperature. Methadone exhibited facilitatory and inhibitory effects at near physiological temperature, and only inhibitory effect at room temperature. Most drugs inhibited the ramp current more potently than the step current—a preference enhanced when Ba(2+) was the charge carrier. The slopes of the concentration-inhibition relationships for many drugs were shallow, temperature-dependent, and differed between the step and the ramp current. CONCLUSIONS AND IMPLICATIONS: All experimental factors examined affected Ca(V)1.2 pharmacology. In addition, whole cell Ca(V)1.2 current characteristics—rundown, temperature sensitivity, and impact of series resistance—are also factors that can impact pharmacology. Drug effects on Ca(V)1.2 channels appear more complex than simple pore block mechanism. Normalizing laboratory-specific approaches is key to improve inter-laboratory data reproducibility. Releasing original electrophysiology records is essential to promote transparency and enable the independent evaluation of data quality.