Phosphorylation of both voltage-gated L-type calcium channels (responsible for the trigger Ca++) and phospholamban, a protein associated with SERCA, is carried out by cAMP-dependent protein kinase. These phosphorylations work together to increase the amount of Ca++ stored in the SR. Specifically, phosphorylation of the sarcolemmal calcium channel leads to an influx of trigger Ca++ into the cell, while phosphorylation of phospholamban enhances the activity of SERCA, allowing the SR to accumulate more Ca++ before it is removed by the 3Na+-Ca++ antiporter and the sarcolemmal Ca++ pump. As a result, more Ca++ is released into the cytosol during the next action potential, leading to an increased interaction between actin and myosin and consequently a stronger contraction force. This increased SERCA activity, induced by sympathetic stimulation, also leads to a shorter contraction duration due to the rapid reaccumulation of Ca++ by the SR. This allows the heart to relax at a faster rate. Additionally, sympathetic stimulation causes an increase in heart rate by directly affecting the pacemaker cells. Stimulation of β-adrenergic receptors in the heart enhances the force of contraction. Electrical stimulation of the myocardium results in a temporary rise in intracellular [Ca++] and force production (A). The β-adrenergic receptor agonist, isoproterenol, amplifies the amplitude of the intracellular Ca++ transient, resulting in a greater force generation (B). Several additional proteins and micropeptides that are associated with SERCA and influence SR calcium transport have been identified. One such protein is the 34–amino acid peptide dwarf open reading frame (DWORF), which enhances the calcium affinity of SERCA (Fig. 13.5) by potentially displacing phospholamban. DWORF was discovered in putative non-coding RNA. The mechanisms underlying the response of the heart to β-adrenergic stimulation are intricate and involve cAMP-dependent phosphorylation of multiple proteins. An A kinase anchoring protein (AKAP) has been found to closely associate with the L-type calcium channel in the heart, thereby positioning cAMP-dependent protein kinase near the channel. This facilitates cAMP-dependent phosphorylation of the channel during sympathetic stimulation. The details of how these cAMP-dependent phosphorylations increase the amplitude of the intracellular Ca++ transient, resulting in a more forceful and briefer cardiac contraction, will be discussed later in general terms. (See also Fig. 13.3)
Both voltage-gated L-type calcium channels (responsible for the trigger Ca++) and a protein associated with SERCA, called phospholamban, are phosphorylated by cAMPdependent protein kinase. The combined action of these phosphorylations increases the amount of Ca++ in the SR. Specifically, phosphorylation of the sarcolemmal calcium channel causes more trigger Ca++ to enter the cell, and phosphorylation of phospholamban increases the activity of SERCA, thereby allowing the SR to accumulate more Ca++ before it is extruded by the 3Na+-Ca++ antiporter and the sarcolemmal Ca++ pump. The net result is that the SR releases more Ca++ into the cytosol during the next action potential, which promotes more actin-myosin interactions and hence greater force of contraction (see
Fig. 13.3 ). The increased activity of SERCA after sympathetic stimulation also results in a shortened contraction because of the rapid reaccumulation of Ca++ by the SR. This in turn allows the heart to increase its rate of relaxation. An additional consequence of sympathetic stimulation is an increase in heart rate through a direct effect on the pacemaker cells (see •Fig. 13.3 Stimulationofβ-adrenergicreceptorsintheheartincreasestheforceofcontraction.Electricalstimulationofmyocardiumresultsinatransientriseinintracellular[Ca++]andproductionofforce(A). Isoproterenol(aβ-adrenergicreceptoragonist)increasestheamplitudeoftheintracellularCa++transientandhencetheamountofforcegenerated(B). A Control B +Isoproterenol Ca Ca Force Force 0.5 sec
Additional proteins and some micropeptides also appear to be associated with SERCA and influence SR calcium transport. This includes the 34–amino acid peptide dwarf open reading frame (DWORF), which increases SERCA calcium affinity (
Fig. 13.5 ), apparently by displacing phospholamban. DWORF was identified in putative non-coding RNA.
Themechanismsunderlyingtheresponseofthehearttoβ-adrenergicstimulationarecomplexandinvolvecAMPdependentphosphorylationofseveralproteins.AnA kinase anchoring protein (AKAP) hasbeenshowntobecloselyassociatedwiththeL-typecalciumchannelintheheart,therebypositioningcAMP-dependent protein kinase closetothechannelandfacilitatingcAMP-dependentphosphorylationofthischannelduringsympatheticstimulation.HowthesecAMP-dependentphosphorylationsincreasetheamplitudeoftheintracellularCa++ transientand,insodoing,resultinamoreforceful,briefercardiaccontractionisdiscussedingeneraltermslater(seealso