Objective: Caveolae, flask shaped invaginations of the cell membrane, influence signalling cascades in many cell types. We have tested the hypothesis that caveolae modulate excitation-contraction coupling (ECC) and beta-adrenergic stimulation in the adult cardiac myocyte.
Methods: Shortening, [Ca(2+)](i) and L-type Ca(2+) current (I(Ca,L)) were recorded in rat ventricular myocytes. Caveolae were disrupted with methyl-beta-cyclodextrin (MbetaC).
Results: Shortening and [Ca(2+)](i) transient amplitude were reduced in myocytes treated with MbetaC. MbetaC did not alter the density or characteristics of I(Ca,L) or the sarcoplasmic reticulum (SR) Ca(2+) load, but significantly reduced fractional SR Ca(2+) release. The inotropic response of myocytes to beta(1)-adrenoceptor stimulation was insensitive to caveolae disruption. By contrast, the increase in shortening, [Ca(2+)](i) transient and I(Ca,L) seen following beta(2) stimulation was markedly enhanced (3-5 fold) following MbetaC treatment, and the effect on I(Ca,L) could be mimicked by dialyzing cells with an antibody to caveolin 3. When the G(alphai) pathway was disabled with pertussis toxin (PTX), control cells showed a similar response to beta(2) stimulation as seen in MbetaC-treated myocytes, whereas MbetaC-treated cells were insensitive to PTX.
Conclusions: Caveolae modulate ECC via the efficiency of the Ca(2+)-induced Ca(2+) release process, rather than Ca(2+) influx. Our data are also consistent with the hypothesis that interaction of G(i) protein cascade components with caveolin in the caveolae is necessary for effective signalling by this pathway. This suggests that changes in caveolin expression in the adult heart seen during aging and in disease will have consequences for baseline cardiac function and beta-adrenergic responsiveness.