14th International Congress
THE "NEW FRONTIERS"
OF ARRHYTHMIAS 2000

Jan. 29 - Feb. 5, 2000
Marilleva, Trento, Italy

RT-30

Roles of b1 and b2 adrenergic receptors in L-type calcium current

Zsolt Nagykaldi, David Kem, Ralph Lazzara, Bela Szabo.
University of Oklahoma, USA

Methods

The methods have been described in detail elsewhere⁶. Isolated canine myocytes were exposed to agonists, blockers, and antagonists of b₁ AR and b₁ AR, a adrenergic receptors (a AR), muscarinic receptors (MR) including isoproterenol (ISO), norepinephrine (NE), epinephrine (EPI), prazosin, carbachol, zinterol (ZIN), atenolol (AT), CGP20712A (CGP), ICI118, 551 (ICI), and modifiers of transduction including phosphodiesterase inhibitors (IBMX) and forskolin. Whole cell I_CaL was determined under standard voltage clamp conditions with buffered cytosolic calcium and blocked K currents (cesium+TEA) and negligible chloride current.

Fig. 1: The effect of zinterol (ZIN = 50 nM) on the non-cumulative concentration-ICaL relationship of (-)-norepinephrine (NE), in the presence of prazosin (PRAZ = 1.0 µM). For comparison, the concentration-ICaL relationship of NE is shown in the absence of other drugs. The treatment with ZIN was started 2 minutes before NE plus PRAZ. Each point represents the mean value of ICaL determined in 3-6 myocytes. The parameters of the concentration-ICaL relationship of NE + PRAZ were: EC(1)50 = 12 nM, EC(2)50 = 323 nM, E(1)max = 235%, E(2)max = 325% of the control. The parameters of "goodness of fit" and "comparison of fits" for the same concentration-ICaL relationship were R2 = 0.9946; abs. sum of squares = 274.3; p << 0.05. The parameters of the concentration-ICaL relationship of NE + PRAZ in the presence of ZIN were: EC50="15" nM, Emax="310%" of the control. The parameters of the "goodness of fit" and "comparison of fits" for the same concentration-ICaL relationship were: R2="0.9900;" abs. sum of squares="457.3;" p << 0.05.

Fig. 2: Non-cumulative concentration-ICaL relationship of isoproterenol (ISO), (-)-norepinephrine (NE), NE + ICI 118 551 (ICI) and zinterol (ZIN). Each point represents the mean value of ICaL determined in 3-6 myocytes. The parameters of the concentration-ICaL relationship were for ISO: EC50 = 3.0 nM, Emax = 335% of the control. For NE there was a biphasic relationship: EC(2)50 = 380 nM, EC(1)max = 170%, E(2)max = 311% of the control. The parameters for ZIN were EC50 = 506 nM, Emax = 149 ± % of the control. The parameters of "goodness of fit" and "comparison of fits" for the concentration-ICaL relationship of NE were: R2 = 0.9976; abs. sum of squares = 133.9; F = 10.47; p = 0.0318.

Fig. 3: The effect of (-)-norepinephrine (NE = 100 nM) on ICaL in the presence and absence of (-)-epinephrine (EPI = 10 nM) and/or carbachol (CC = 100 nM). Data obtained in 8 myocytes for NE, for EPI, in 5 myocytes for NE + EPI, and in 4 myocytes for NE + CC and NE + EPI + CC.