Phosphodiesterases 3 and 4 regulate local Ca2+ releases and beating of pacemaker

Lakatta, Edward

Abstract

Spontaneous beating of rabbit SANC is controlled by rhythmic, local subsarcolemmal Ca2+releases (LCRs) from sarcoplasmic reticulum which occur during diastolic depolarization. Spontaneous, rhythmic LCRs activate inward Na+/Ca2+ exchange current, imparting an exponential increase to the later part of the diastolic depolarization to fire an action potential. Ca2+ cycling in SANC as well as LCRs are critically dependent on Ca2+ influx via L-type Ca2+ channels which is regulated by high basal PDE and PKC activities. The extent to which either PKC or PDEs regulate basal Ca2+ cycling in SANC as well as L-type Ca2+ current, ICa,L, however, remains enigma. We determined that there is PKC and PDE subtype-dependent control of both basal Ca2+ cycling and ICa,L as well as spontaneous SANC firing rate. Specific PKC inhibitor GF109203X markedly suppressed spontaneous SANC firing, LCR size (from 5.8 0.3 to 2.8 0.3), number per each spontaneous cycle (from 1.4 0.2 to 0.6 0.3) and ICa,L amplitude (from -8.1 1.2 to -4.6 0.9 pA/pF), strongly suggesting Ca2+ cycling could be a major target of basal PKC activity in SANC. All effects of GF109203X were reversed upon washout. A specific PDE4 inhibitor, rolipram, had no effect, on either ICa,L or spontaneous beating;cilostamide, a specific PDE3 inhibitor, in contrast, increased both ICa,L and spontaneous SANC firing. Simultaneous inhibition of PDE3 and PDE4 by (cilostamide+rolipram) increased ICa,L and LCR size (from 5.9 to 8.6 km);decreased the LCR period (from 309.7 to 214.3 msec);and accelerated spontaneous SANC firing rate equivalent to broad-spectrum PDE inhibitor, IBMX. Thus, concerted PKC and combination of PDE3 and PDE4 activities control basal SR Ca2+ cycling and ICa,L amplitude, modulating LCR parameters and spontaneous SANC firing rate.