Calcium channels are ganging up in the sarcolemma.

In this issue of Circulation Research, Navedo et al1 provide provocative and exciting data, indicating that up to 6 L-type Ca channels (LTCCs) (in this case Cav1.2) can physically interact with each other to gate in a coupled manner, producing large and long-lasting Ca influx into smooth and cardiac muscle myocytes. This coupling appears to be relatively rare and transient, with a minority of overall LTCCs involved at any moment. However, coupling is favored by either displacement of calmodulin (CaM) from its binding location in the C-terminal LTCC domain, activation of protein kinase (PK)C , chronic hypertension (in vascular smooth muscle), and LTCC mutants that are associated with cardiac arrhythmias and hearing loss in humans (ie, Timothy syndrome, or long QT syndrome 8). The fundamental observations here were made using total internal reflection fluorescence microscopy by measuring local submembrane Ca events termed Ca sparklets.2–4 These are attributable to Ca entry via LTCC, unlike the sarcoplasmic reticulum (SR)/endoplasmic reticulum Ca release events known as Ca sparks or puffs (mediated by ryanodine or inositol 1,4,5 trisphosphate [InsP3] receptor channels). Physiologically Ca sparklets would be extremely small and brief events, because normal LTCC openings are 1 ms in duration, and current amplitude is only 0.2 pA (at physiological [Ca ]o) resulting in an integrated Ca influx of only 0.1 femtocoulomb (or 300 Ca ions).5 This would produce a very small hard to detect fluorescence signal. However, the Ca sparklets here are much larger, which enhances detectability. They are larger because they are attributable to more rare persistent openings (like mode 2 openings favored by the LTCC agonist Bay K 8644),6 elevation of [Ca ]o, and measurements at negative voltages (where driving force is high). This group has previously shown using patch clamp that these Ca sparklets are indeed caused by LTCC current (not SR Ca release), can last hundreds of milliseconds, and are favored by PKC activation.3 Total internal reflection fluorescence imaging has the advantage (for these events) that one can sample the entire surface of the cell simultaneously, rather than just a small patch of membrane in cell-attached patch-clamp recording. Here, the authors show that local Ca sparklets and patch-clamp currents show quantal increments of 2 to 6 times the normal [Ca ]i and single Ca 2 current amplitude and that the apparent gating transitions of each channel occur essentially simultaneously (ie, coupled gating).1 This raises 3 obvious questions: (1) are these coupled gating events real; (2) why might they be of functional importance or relevance; and (3) how are they regulated?